A Little Help For Eico's HF-81

I use rubber bibb washers (in the plumbing section) to isolate the transformers from the chassis.
 
Reducing Output Stage Sensitivity to Noise

In the last post, the effort was about reducing power supply noise supplied to the amplifier from the get-go. This time, the effort is to make the output stage less sensitive to the noise to begin with -- this because the output stage receives the least amount of filtered B+ voltage in the design. This effort can have additional benefits as well.

From a theory standpoint, the output stage will be least sensitive to power supply noise if the output tubes are perfectly balanced with respect to current flow. The B+ voltages are applied to the output stage in a "common mode" fashion, so that both output tubes of the push-pull circuit receive the same power supply noise that rides on top of the B+ voltage. Since both sides of the push-pull stage receive the exact same noise signal then, the noise component will cancel out in the OPT if the output tubes are perfectly balanced with respect to DC current flow.

In the original design, Eico counted heavily on this basic principle of common mode cancellation through balanced operation. They did not go so far as to specify that the output tubes be matched pairs, as this was first and foremost a kit unit with compromises made to maintain economy. But there is no doubt however that the use of matched output tubes will certainly help towards achieving that end.

To minimize output stage sensitivity to power supply noise then, the tubes must operate with a balanced current flow through them. But balancing the quiescent current flow in the output stage has two other benefits as well, one of which is quite significant.

For starters, when the DC current flow is balance through both halves of the output transformer, not only is is very immune to common mode power supply noise, but it also makes the OPT less susceptible to magnetically induced noise as well.

In the HF-81, the power transformer is quite large in comparison to the OPTs, so there is plenty of magnetic noise impressed upon them. The closest OPT is turned 90 degrees to help minimize any coupling effects. But with so large a power transformer, some coupling can still take place -- and it does. Just give a listen close up to the speaker of Channel 2 when the unit is first turned on. Before the tubes warm up, you will hear the hum of magnetic coupling taking place between the power transformer, and this closest OPT. In the subject factory built HF-81, this OPT was mounted well off of perpendicular to the power transformer, which allowed the OPT to then pick up more noise than an optimum mounting would. For this OPT then, proper orientation is important. Channel 1's OPT is mounted with the same magnetic orientation as the power transformer, so the only help it receives is from being mounted at some distance from the power transformer.

In both cases however, having the DC current flow balanced through each half of the OPT primary winding biases the winding to a higher energy level than that of the available magnetic noise from the power transformer, making the winding rather immune to the magnetic noise impressed upon it. How well balanced the current flow is determines how immune the winding is to the external magnetic noise impressed upon it.

From an audible standpoint however, providing a balanced quiescent current flow through the OPT always enhances the low frequency performance of the OPT. With balance current flow, LF distortion drops to a minimum, and maximum LF power transfer is obtained. When the currents are not balanced, the magnetizing effects in each half of the primary winding don't effectively cancel each other out. Depending on how great the mismatch is, this can cause the core of the transformer to saturate earlier than it would with an otherwise balanced condition. Therefore, balancing the quiescent current flow through the OPTs not only makes them have greater immunity to common mode injected and magnetically coupled noise, but also allows them to achieve optimum LF performance as well. The OPTs of the HF-81 have a bigger core than many EL84 based amplifiers, but are hardly the biggest by a long shot. Balancing the quiescent current flow in them then can have a significant impact on their LF capabilities.

But there are other benefits to balanced operation also. Invariably, in push-pull output stages, one tube begins to become a current hog, drawing more of the current, which in common cathode resistor cathode biased designs, inherently causes the other tube to draw less current. This causes the hog tube to wear out faster, and even potentially over dissipate itself which can lead to its demise even faster. As a result, one popular modification is to raise the value of the cathode bias resistor to lower the overall dissipation levels in the output stage -- not only to account for today's higher line voltages -- but further to the point that even if one tube becomes a hog, it won't go chernobyl in the process.

Raising the cathode bias resistor slightly to account for higher line voltages is fine, but raising it further to account for the "what ifs" only serves to reduce power output and increase distortion. Therefore, maintaining a balanced current flow through the output stage also serves to allow maximum performance from the output stage, while achieving maximum tube life from it as well.

So while balancing output stage quiescent currents produces many benefits, Eico didn't provide any means to accomplish this other than through the use of matched output tubes -- which are rarely "identically" matched when new, or if they are, rarely stay so over time. Therefore, some means of accomplishing balanced output stage operation would be a very worthwhile improvement for the HF-81, with positive impacts on hum, LF performance, and tube life.

A suitable DC balancing circuit was then devised, along the same lines as those used by Williamson in his original amplifier. I modified his basic design with component values as appropriate for EL84 type tubes, and improved it by including matched 10 ohm 1/4 watt resistors in each cathode leg, which allows a simple voltmeter to be used to adjust for a 0.00 volt difference between two test points (the two cathode terminals), indicating perfect quiescent balance. This is similar to the same approach Heath adopted in the W-5M, but only requires one cathode bypass cap. The 1/4 watt resistors not only allow for an easy DC balance adjustment to be made, but also provide protection for the OPTs should a tube decide to become socially unacceptable.

A terminal block was mounted atop the rear of the chassis to effect the test point connections, allowing for easy balance adjustments to be checked or made, without removal of the bottom cover. Channel 2's balance control occupies the former position of the hum balance control (which is why it was moved), while a new mirror image hole was drilled to accommodate the balance control for channel 1.

Pics include:

1. The output stages modified to include DC balance capabilities in each channel. The stock value cathode bias resistors were maintained, while additional resistive elements of the balance circuits effectively raise this value to about 185 ohms. This causes the tubes to operate at a conservative 80% of the Design Center dissipation rating for the tubes.

2. Top side, the two DC balance controls are readily apparent, as is the terminal block at the rear of the chassis to allow for easy meter connections to make the adjustments. The shielded cable to the panel lamp is also visible in this pic.

Next up, quieting down the line/tone amp stages.

Dave
 

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Hi Dave, I'm curious, did you account for the failure condition that can develop over time where the wiper of the balance pot gets dirty and might intermittently disconnect from the circuit?

Might be okay though if that happens--in looking at the original Williamson schematic, if the balance pot wiper does disconnect, it would shut down all current through the tubes, basically turning the tube off, which I guess is way better than the other extreme--loosing bias and red plating.
 
Nice clean mods. Too bad I'll never get to hear it. When do we get to see the PS? I'm curious about the screens and front end power.
 
Dave- it'd be interesting for us to try that balance circuit on our bigger Eico HF amps HF22s and/or HF35s)...

I'm thinking that would even further reduce the measured distortion on my HF22s, that we tested before...
 
Line/Tone Amp Stages

In the Eico equation of all things audio, they invariably placed the volume control at the input of the line and tone control amplifier stages in virtually all of their products, and the HF-81 is no exception to this rule. The benefit of this approach is that it basically becomes impossible to overload these stages, since the volume control attenuates the signal before being applied to them. On the downside however, it means that these stages are running flat out wide open gain wise, not only sending all of their own noise to the power amplifiers to amplify, but also any extraneous hum or noise signals they might pick up, amplify, and send along for the ride. It is this basic design equation that has given Eico products their reputation for often having excessive quiescent hum and noise.

In the HF-81, the total noise generated by the line/tone amplifier stages comes from three sources:

1. The normal flow of electrons moving through electronic components. This produces the normal rushing sound making up a portion of the quiescent noise level.

2. The AC voltage running through the close proximity heater wiring and the heaters of the 12AU7 tubes used in these stages. The product of this is good old 60 Hz hum, enhanced by only partially bypassing the cathode resistors of these stages. The hum balance control can null this hum out to some degree, but the optimum setting for one channel rarely coincides with the optimum setting for the other.

3. The significant magnetic noise from the power transformer that is distributed at various levels across the chassis.

Minimizing the first source is simply a matter of using quality components -- namely low noise plate and cathode resistors, and of course tubes. Beyond that, there is little that can be done to reduce this noise level, short of a change in circuit topology. As mentioned earlier however, this noise element is not particularly problematic in the HF-81, so the normal measures suffice to control this source of noise.

The second source will be the subject of this post, while the third source will receive its own post coming up, which is dedicated to that subject.

With the previous work done to the B+ power supply section, it freed up the dedicated 6.3 vac 2A winding in the power transformer that was originally used to power the heaters of the rectifier tubes. Initially, this winding was going to be used to buck the AC line voltage to return operating voltages back to normal when the unit was powered from a typical 121 vac power line that is common today. However, when the quiescent noise level of the unit became a concern, those plans were abandoned, and the line voltage adjustment was then supplied by a single CL-90 current limiting device that was added to the primary side of the power transformer. This worked well, not only reducing the AC line voltage appropriately, but providing surge protection as well. As stated earlier, because of the proper installation of the new SS rectifiers, surge protection isn't really needed in this unit. But it is a nice feature none the less in addition to the line voltage adjustment provided.

With the rectifier heater winding now free, it becomes the perfect source to power a DC supply for the heaters of the line/tone amp stages, and the phono preamp stages as well. This particular unit will have the phono preamps disabled, but a supply powered from this winding is capable of powering them as well just the same.

The most logical approach is to use this winding to power a full wave voltage doubler rectifier configuration, which provides over 15 vdc at its output. This is high enough to then filter through an appropriate dropping resistor (10 ohms in this case) and following filter cap, to supply a quiet source of 12 vdc power to operate these heaters. In a worst case scenario (powering line/tone and phono preamp tubes), 0.6 A of current capability is needed, which is doubled of course by the use of a voltage doubler rectifier circuit. This is further raised by the charging current in the caps of the doubler circuit, but still, the total current draw is less than that originally drawn by the heaters of the rectifier tubes. What's more, this power is virtually free with the exception of the charging current drawn by the voltage doubler filter caps, because the current drawn by the heaters from this winding is no longer being drawn from the main 6.3 volt heater winding. Therefore, it is basically a swap of heater current draw from one winding to another.

This is a far superior way of providing DC voltage to power the small signal tube heaters than trying to rectify the power from the main heater winding for that purpose. That 5A winding is already operating at maximum capacity in the original design, so adding the charging current drawn by the filter caps of a DC supply connect to it would then significantly overload the winding. With the approach taken, current draw is significantly reduced on the 5A winding, while current draw on the 2A winding is less than in the original design.

No pics are provided as the efforts of this modification can be seen in the two pics provided in post #8.

In the underside shot, you can see the voltage doubler rectifier circuit and dropping resistor built onto a T-strip located at the bottom left corner of the power transformer in the shot, while in the top side shot you can see the filter caps for this supply. The doubler caps are the two caps located together, while the output filter cap is the single cap located over the choke. Ripple at the output of this supply is < 100 mv, so for heater power, it is quite clean indeed.

The efforts so far have completely eliminated all audible effects of 120 Hz ripple from the B+ power supply, and 60 Hz hum from the audio circuits as well, thanks to clean DC power now operating the heaters of the small signal tubes. Performance of the output stage has been optimized in the process, output tube life has been enhanced, dependability increased, and the unit operates cooler as well. This is a greatly improved HF-81, and the design of the basic HF-81 audio circuits hasn't even been touched in the process, either. So while the hum has been significantly reduced at this point, still, some hum persists, and it comes from a source that is particularly devilish to deal with -- next time.

Dave
 
Thanks for your ongoing work in this post. It shows that tackling hum requires systematic thinking through and dealing with the causes, and is not just a matter of lead dress. Now for the PT!
 
I guess I'll be printing and filing this. My old ears must be worse than I thought. I felt pretty good about the last one I sent out. I test for noise with my ear against Heresy horns. It sucks getting old, and deaf.:tears:
 
Line/Tone Amp Stages, and the Elephant in the Middle of the Room

So finally, we come to the worst offender of all -- the power transformer. While Eico typically includes a static shield in their power transformers, such shields are typically only effective at minimizing line noise from getting through the transformer. This is the separate white lead associated with Eico power transformers that gets grounded to the chassis.

What Eico power transformers don't have, is a Faraday shield -- other than in their oscilloscope kits, and that for rather obvious reasons (to prevent distortion of the image on the screen). A Faraday Shield is a wide copper strap that runs perpendicular to the laminations, under the end bells, outside of the coil form, making a complete loop around the transformer, and serves to significantly reduce the amount of magnetic energy escaping from the transformer. Sometimes its (obviously) copper in color, and sometimes it's painted the color of transformer depending on how the transformer was manufactured. But when included, it is unmistakable in appearance, as well as in how effective it is in controlling radiated magnetic noise into the nearby circuits.

To make matters worse however, the HF-81's power transformer doesn't even include a bottom end bell. And, it's mounted through hole style in the chassis, so that the open windings appear below the deck to flood the area with unrestricted noise. It's the equivalent of the loudmouth in the middle of the room that nobody can get far enough away from.

The final nail in this coffin, is that it's mounted of course on a substantial steel chassis, that easily conducts all the copious amounts of magnetic energy radiated from the transformer through out the ground plane of the audio circuits. It simply can't get any worse than this.

It will be found that the magnetic energy in the chassis spreads throughout it in a decaying fashion, so that the strongest energy is (of course) nearest the transformer, and gradually reduces itself the further away from the transformer you get. Understand however that this magnetic energy takes on two distinct forms:

1. As an electrical energy representing different potentials at different points throughout the chassis, and

2. As a magnetic field running along the ground plane (chassis), engulfing the components and wiring at different levels throughout the chassis.

This is insidious stuff, that challenges everything you think you know about best grounding practices, as will be shown.

Eico used a single snake ground lead for all of the line/tone amp stages, as well as the input stage of the power amplifier sections. This snake ground is ultimately grounded to the chassis at a grounding lug over near one of the phono preamp stages, and works it way back across the chassis through the line/tone amp stages, visiting various T-strips along the way, and ultimately ending at a T-strip located a fraction of an inch away from the (former) inside rectifier tube location. This lead has plenty of noise in it that it picks up from the magnetic field running through out the chassis. The effects of this are rather easy to show.

With the HF-81 running, use any convenient amplifier as a signal tracing amplifier with a shielded cable for its input lead. Ground the shield at the grounding lug where the snake ground lead connects to the HF-81 chassis. Ground the hot end of this lead at the other end of the snake lead, and turn up the gain of the signal tracing amplifier. The hum will be significant. Heck, do it the other way around: Ground the shield at the end of the snake ground lead, and connect the hot to the chassis ground lug, and you get? Same ol' hum! This doesn't take some high gain amplifier to hear, either. Any amplifier with a 1 volt input sensitivity will do just fine. Understand that the presence of this hum signal along the snake ground lead then proves that the various stages connected to it, are operating at -- or being stood on -- different levels of ground noise that occurs at different points along the snake ground lead. If you use heavier ground leads, or individual ground leads to the common point, you are in fact lowering the impedance of the ground system, but also making the ground system a bigger antenna to pick up the magnetic field energy flowing throughout the chassis. As a result, little changes.

The take-away point from this exercise is this: Even with a common ground point for the circuits, if the circuits are physically located at one point on the chassis, but the chassis ground for these circuits is located at another point on the chassis, then the field energy flowing between these two points in the chassis will induce noise into the ground wiring and the components of these stages, which effectively becomes a hum signal injected into them, is amplified, and presented as hum in the output. Therefore, the lowest INDIVIDUAL stage hum level is actually achieved by grounding each stage in the immediate area where the stage is located.

Such an approach eliminates any ground wiring, and places the components of each stage over the ground plane at a point that has the same energy level that the ground circuit represents. Of course, while this works on an individual stage basis, it does not work in the whole of the HF-81 chassis, where all the stages must interact with each other: Grounding each stage at separate points on the chassis only acts to insert the chassis noise between the various stages, making stages that are individually very quiet, quite noisy when connected together.

In the end then, a modified snake ground line proved to inject the least amount of magnetic noise into the ground system and the components scattered over surface of the chassis.

However, Eico could have optimized this approach better, by considering WHERE various ground points were made along the original snake ground line. For example, in designing the ground circuit for the tone stage wiring of channel 1, they connected the cathode components of this stage right to the chassis ground lug. But the grid return of this stage (tap on the treble control) is made at the other end of the snake ground lead. This amounts to directly injecting the full noise developed along the snake ground lead system into the grid of the tone amp stage for channel 1, as a signal for it to amplify, and present as hum in the output. The same thing is done -- to a lesser degree -- with the tone stage of channel 2. The point is, for the least amount of hum from magnetic noise in the line and tone amplifier stages, the ground point for the grid and cathode components of each stage MUST occur at the exact same point along the snake ground lead, or else the noise potential that exists between differing ground points will then be injected into the stage, and amplified as hum in the output.

The bottom line of all of this, is that while ensuring that grid and cathode ground points for each stage occur at the same point will in fact help lower hum from this section of the amplifier, the ultimate hum noise floor is still established by the amount of magnetic noise in the chassis, which there is simply no way around.

Various different scenarios were tried that literally had the two separate channels grounded at two separate places, with each place representing the most localize location within the area where a channel's wiring resided. That produced some of the lowest noise levels observed -- until of course a stereo source was plugged in that had a common ground between the two channels. When the volume was advance but no signal present, the ever present ground loop this formed made itself heard quite readily. There is simply no way around the noise created by HF-81's power transformer, and the differing field energy levels it produces throughout the HF-81's chassis.

Understand however that all of the measures presented throughout this thread have in fact improved the HF-81's signal/noise ratio quite substantially: What struggled before to barely achieve a noisy spec value of -75 db below rated output (which I doubt that many of the kits ever achieved), now achieves a much more tolerable -84 db below rated output. In fact, in a quite, smallish room with my 101 db Cornwalls, the noise level -- which before was completely unacceptable -- is now basically inaudible from my listening position.

The pic included shows the revised wiring of the snake ground system in the tone/amp stages: Each channel has it's own snake now, but the snakes are both grounded at the same common point on the chassis. This helped to prevent intermixing of ground noise in the ground wiring, produced by each channel being physically located at different points on the chassis. Also, the cathode and grid grounding points for each stage occur at the same point on each snake ground now, eliminating any directly injected ground noise into each stage. Finally, additional shielded cable was used for sensitive leads, and the center pins of the line/tone amp stage tube sockets are now grounded. Also visible is the twisted wiring carrying the 12 vdc to power the heaters of these tubes, now wired for 12 volt operation. Collectively then, these measures have made a significant dent in quieting the hum generated from these stages. Notice too the new Alps volume control snuck in there, replacing the original device.

What remains now is a greatly reduced and tolerable noise level that is comprised of equal parts hiss, and subdued magnetic hum, which manifests itself as a sort of raspy 120 Hz hum, that can be heard within 2 or 3 feet of midrange horns -- which is where the project stands at this point...........

But it ain't over, yet!

Dave
 

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Dave, would a different PT with a Faraday shield, located in the same place on the same steel chassis avoid this hum issue?
 
Dave, would a different PT with a Faraday shield, located in the same place on the same steel chassis avoid this hum issue?

I'm trying to remember if the Heyboer replacement was shielded and don't have one here. Of course I can't remember, but I don't "think" they are.
 
Good stuff here Dave.

The point is, for the least amount of hum from magnetic noise in the line and tone amplifier stages, the ground point for the grid and cathode components of each stage MUST occur at the exact same point along the snake ground lead, or else the noise potential that exists between differing ground points will then be injected into the stage, and amplified as hum in the output.

It's easy to forget that the same principle that amplifies for audio signal reproduction (difference in voltage between grid and cathode) can also amplify for noise reproduction!

Various different scenarios were tried that literally had the two separate channels grounded at two separate places, with each place representing the most localize location within the area where a channel's wiring resided. That produced some of the lowest noise levels observed -- until of course a stereo source was plugged in that had a common ground between the two channels. When the volume was advance but no signal present, the ever present ground loop this formed made itself heard quite readily. There is simply no way around the noise created by HF-81's power transformer, and the differing field energy levels it produces throughout the HF-81's chassis.

So I assume you compromised for a given amplification stage and grounded both channel's cathode ground point and grid ground point to the same location on the snake ground system?
 
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Really interesting read,,, I've tried to copy like sections of a build, channel for channel, in the same spot on the ground bus,,, Never had any problem with hum or noise,,, Its the rest of it I've been struggling with!!

Thanks again for the lesson...

Regards,
John
 
Grand Finale

This project has had a number of separate improvement efforts within it, of which the hum reduction effort was just one. Collectively, the improvements then amounted to:

1. Doing a complete make over of the RCA jack layout over that of the original design, providing 4 separate high level stereo inputs. Because the low level inputs were not going to be used on this unit, it freed up that jack area to help facilitate this goal (all the low level circuits were to be left in place and untouched as much as possible, but otherwise their input jacks and wiring removed, and power to these circuits removed as well). Cardas jacks and bare FR4 board made for a neat, clean revision of the jacks, with left and right inputs clearly marked by colored rings now around each jack. The FR-4 allowed the jacks to remain isolated from the chassis for lowest hum and noise.

2. The same treatment was given to the old Tape Output jacks, with FR-4 and Cardas jacks installed in this location as well. However, these jacks are now slightly reconfigured in conjunction with the old top chassis service switch. That switch now serves as a Preamp Output Switch, sending the preamp output signal to either the internal power amps, or out the old Tape Output jacks to serve as a Preamp Output feature. When the Preamp Output option is chosen, the power and phase inverter tubes may simply be removed, and the unit then operated as is a a full featured preamp, with most of the same features as that of the HF-85 preamp -- without any of the low level circuits of course. The switch prevents any external load connected to these jacks from affecting the internal signal path, or prevents the internal power amps from affecting the signal presented to the new Preamp Output jacks.

3. In finishing out the rear deck connections, the decision was also made to make for a more logical arrangement of the output connections as well.

Eico engineers were often too cute by half when it came to implementing the features of their equipment, many times placing themselves at odds with the majority of other manufacturers in this regard. Whereas others used channel designators of L&R or A&B, Eico used 1&2 for that purpose, reserving A&B for input options, where others used 1&2 for input options. The high level input jacks on the HF-81 are not laid out with any symmetry, with an odd number of input jacks, and one jack performing dual service serving one channel on one function, and the other channel on another function. During the days of AM-FM Stereo broadcasts and fledgling FM Stereo MPX, this was a useful feature, but most often today, it just comes off as confusing. To top it all off, when looking at the rear panel of the HF-81, the inputs were generally laid out with left channel inputs on your left, and right channel inputs on your right. However, this moment of clarity in the Eico think tank completely exploded when it came to output connections, with the left channel speaker connections and Tape Output jacks appearing on your right, and the right channel outputs appearing on your left.

To emphasize the level of utter chaos this general level of thinking achieved within the halls of Eico, the schematic of the HF-81 as published by Eico is only accurate if you understand that the upper phono preamp circuits are for the left channel, while the upper line, tone control, and power amp circuits are for the right channel!!! What were these guys thinking??

In trying to regain some sense of reality then, as mentioned previously, installation of the new Cardas RCA jacks immediately resolves the channel identification crisis for ALL of these jacks, while reassigning power amplifier channel identification now lets the left channel speaker terminal board be on your left, and the right channel terminal board be on your right. Consistency reigns for all connections now. What a concept!

4. Also as previously mentioned, the power switch is now located where the old Tape Speed switch used to reside -- minimizing wear and tear on the right channel treble control where the original switch was located (since it will no longer need to be rotated now to turn the unit on and off), and reducing hum by removing the AC power switch from the sensitive nearby tone control circuits.

5. All components have now been matched between the channels, if they weren't already. Performance has been verified to be virtually identical between the channels now, so that adding the Alps volume control allows for a very focused sound stage at all control settings now. Also, all the output tubes now idle at a very comfortable 10 watts dissipation each, with the tubes in each channel balanced for equal current draw by way of the new facilities to achieve that state.

6. All of the previous power supply and DC balance facilities have already been detailed to enhance performance, and reduce noise.

These efforts have really put this HF-81 in an elevated class of contenders now, with it not only very enjoyable to listen to, but to use and interact with as well. But still, that final layer of hum remained. What was annoying about it, was that it was no longer the low muffled tone of 60 Hz hum that covered up the other hum noise, as it had all been eradicated. What remained was that final low level, almost raspy, but undeniable hum that occurs from the magnetic energy running throughout the chassis, that could not be removed unless a way could be found to build all the circuits of both channels one top of each other at one point on the chassis. Since that's not possible, then the next best thing had to happen: The power transformer had to go.

Actually, all of the previous work is what made removing the transformer sooo successful, and, fairly easy to do from a stand point of execution.
Because all of the audio and power supply circuits had now been improved and/or reinstalled in a lowest possible noise fashion, removing the power transformer amounted to just that.

Normally, when a power supply is remoted, it not only includes the power transformer, but the rectifiers, filter circuits, and associated circuitry. In this case, literally, just the power transformer was removed, with all the rectification and filtering circuits remaining on board the HF-81. The results of remoting the transformer are really eerie as compared to the on board installation.

Normally, when you turn on a stock HF-81, the hum level goes through four phases rather quickly:

1. You immediately hear the magnetic coupling in both speakers, but particularly the one connected to the OPT closest to the power transformer.

2. As the output tubes begin to conduct, the magnetic coupling noise is diminished, replaced by 120 Hz hum from unbalanced output tubes.

3. As (or if) the output tubes start conducting more equally, then the 120 Hz hum is reduced, and the magnetic energy flowing in the chassis and amplified by the power amplifier circuits then begins to dominate and remain.

4. As the 12AU7 line stage tubes heat, then the magnetic hum is enhanced, and the unmistakable sound of 60 Hz AC heater hum comes in as an underlying foundation to the total hum noise.

With all the improvements already made and now the power transformer removed, when you turn on the amplifier, you get:

1. Initially.....nothing -- in either speaker. Dead silence.

2. As the output tubes begin to heat, you get......nothing -- whether the output tubes are balanced, or not. Still dead silence.

3. As the power amplifer circuits warm, you get......nothing.

4. As the 12AU7 tubes heat, you hear a soft hiss fade in as the tubes warm.

Turn the unit all the way up, advance the bass control to full, inputs connected or not, it doesn't matter. All that does is add to the timber of the hiss. It's doesn't bring out any otherwise muted hum noise. And all of this is with all the covers and skins off! There is literally ZERO hum in this unit now.

When the initial test of removing the power transformer produced such absolute results, the obvious move was made to properly remote the transformer. This portion of the project is still underway, but the pics provided will show the process and expected outcome:

1. This was a highly controlled test, with no expense spared to see if getting the damn thing out of the chassis would achieved the desired goal. It surpassed all expectations.

2. A suitable plate was then fashioned to cover where the transformer used to reside.

3. The power supply area was reworked slightly to accommodate the remote mounting of the transformer. The new front panel AC power switch on the HF-81 chassis is still active, as is the non-switched AC outlet on the rear apron. The umbilical is a 10 conductor 16 ga umbilical of nearly 6 feet in length, that is permanently attached to the HF-81 chassis. These efforts ensures no loss of voltage, particularly through the important heater leads.

4. On the new "power supply" chassis will reside the transformer, fuse, and CL-90 current limiter. Also there will be a standard IEC power cord connector, and 12 pin female Jones socket to accommodate the 12 pin male Jones plug on the umbilical. This arrangement ensures that no shock hazard exists, since AC power enters the power supply chassis first.

5. All parts have arrived except for the Jones plug for the umbilical. Therefore, the amplifier sits waiting and ready, with all skins back in place. The power supply chassis should hopefully be done tomorrow, with only the umbilical plug remaining.

The hum was very tolerable before the transformer was removed. But for those who want the last word in a black background, removing the power transformer is the only way to go. Just remember however, that it was but one of the required steps to finally eliminate all of it.

Dave
 

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A choke input configuration is quiet no doubt, although it takes a much larger choke than the one installed (inductance wise) for the choke input filter to work properly. A swinging choke would be best, but in any event, you'd also lose about 100 volts of B+ in the process. The current configuration is producing just under 100 mv of ripple at the OPT CT connection, which is extremely quiet compared to the 6+ volts of ripple in the stock design at that point, and produces no audible noise in the speaker after cancellation in the push-pull connection.

Dave
 
What's the rationale for NOT relocating the rest of the PSU? Is it just more work than necressary or some technical reasons?
 
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