Apt Holman Preamplifier Tone Switch Pop

[goldswimmerb]

Hey all, I have an APT Holman Preamplifier I recent recapped and gave some new transistors as the 2SC1345s had gone noisy.

I have it all back together now and everything is working beautifully, however, if I switch the tone defeat switch on/off the unit gives out a fairly decent bit of DC causing a pop. It only does it right when the switch is engaged, when in normal operating conditions there isn't significant DC at the terminals. I noticed that it didn't look like they had a snubber/DC supression at the switch, so I was wondering if this behavior is normal or something I should worry about?

Every other function is working beautifully.

 

[pwauk12]

There should not be a pop or DC in normal operation. Those switches can give problems and I suggest you replace the switch.

I think an NKK switch will work, - Mouser part 633-M2022SA1W40

 

[goldswimmerb]

There should not be a pop or DC in normal operation. Those switches can give problems and I suggest you replace the switch.

I think an NKK switch will work, - Mouser part 633-M2022SA1W40

I don't necessarily think a switch would cause DC offset to spontaneously appear, contact resistance checks out good.

 

[pwauk12]

this can occur with this switch, what level of DC are you measuring on the output?

 

[goldswimmerb]

this can occur with this switch, what level of DC are you measuring on the output?

It's faster than my multimeter, but I can briefly catch it on my scope, it's somewhere between 200mv and 500mv though, causes a nice pop

 

[pwauk12]

I am guessing you replaced C32/C132 and C68/C168?
Is it on both channels?
I do think the switch contacts are the problem, I have had similar issues on the many of these I have worked on and a new switch was nearly always the answer. If it is not the switch, then I ma unsure how the DC would get to the output only when the switch is operated.

 

[goldswimmerb]

I am guessing you replaced C32/C132 and C68/C168?
Is it on both channels?
I do think the switch contacts are the problem, I have had similar issues on the many of these I have worked on and a new switch was nearly always the answer. If it is not the switch, then I ma unsure how the DC would get to the output only when the switch is operated.

A bit more on the right than left. I might try swapping switches around to see if it makes a difference.

 

[goldswimmerb]

I am guessing you replaced C32/C132 and C68/C168?
Is it on both channels?
I do think the switch contacts are the problem, I have had similar issues on the many of these I have worked on and a new switch was nearly always the answer. If it is not the switch, then I ma unsure how the DC would get to the output only when the switch is operated.

Turns out the tone IC was bad, was putting out .4v DC on one channel, it was passing audio fine and looked okay on the scope, swapping it for an OPA2134 worked perfectly and lowered my noise floor. If I had a larger budget to work with I'd be swapping all of them out.

 

[pwauk12]

Great that you found the problem. If possible replacing all the IC's with 2134, except the volume one (TL2072), will definitely reap benefits. There is a review on audio science review of an APT that I upgraded with this configuration.

They are great preamps

 

[goldswimmerb]

Great that you found the problem. If possible replacing all the IC's with 2134, except the volume one (TL2072), will definitely reap benefits. There is a review on audio science review of an APT that I upgraded with this configuration.

They are great preamps

I would love to try it, unfortunately, this unit is from a shop I take work for and they don't want to spend the money if they don't have to. I'm certain it takes this preamplifier to the next level though.

 

[pwauk12]

I know exactly what you mean- a shame really, but it is what it is

 

[HalfApt]

Great that you found the problem. If possible replacing all the IC's with 2134, except the volume one (TL2072), will definitely reap benefits.

I read one other post long ago regarding the TL072/TLE2072 only for volume control, but have never heard the reason why.

Do you have an explanation/link?

 

[pwauk12]

According to the the manual and the designer the volume control circuitry controls the gain of an inverting amplifier to keep any noise to a minimum, with the IC and Q10/Q11 forming a feedback loop do as the volume is decreased so is the gain. The volume control is a 32 step attenuator with differing attenuation steps ranging from smaller to larger as the volume is increased. As I understand it, the wiper of the volume pot is held at zero volts by the IC, has no signal on it at all. Consequently the replacement of the original TL072 is the modern lower noise TL2072 as the IC is not handling audio.

Like many things in this preamp, it is a very clever approach.

 

[mcgjohn]

Turns out the tone IC was bad, was putting out .4v DC on one channel, it was passing audio fine and looked okay on the scope, swapping it for an OPA2134 worked perfectly and lowered my noise floor. If I had a larger budget to work with I'd be swapping all of them out.

We used to work on these preamps. There were a few opamp failures, but not many. We finally soldered in sockets so the owner could swap out the opamps if better units became available.

Completely agree with your thoughts on swapping them all out. A lot of developments in opamps have occurred since the TL072 was designed.

Besides all the opamps, there are two main weak points on the apt preamp.

1. the tiny 12 watt power supply.
2. all the interstage electrolytic coupling caps used in the audio signal path.

Its not too difficult to bypass all the electrolytic caps with a .33 or .47 film cap which helps immensely.

the transformer requires a bit more effort.

Its been quite a long time, but think the dual secondaries are 22VAC on the stock unit. We would add an outhboard 200 watt to 400 watt toroid power transformer in a smaller case. Then run the secondary supply lines in to the chassis and solder them directly where the stock leads were attached. Would leave the stock trans in place but disconnected.

Holy cow those little apt preamps really came to life when doing this.

The parts required to do so are not that expensive. But there is some time involved in selecting the right case and transformer and connecting everything up.

 

[HalfApt]

According to the the manual and the designer the volume control circuitry controls the gain of an inverting amplifier to keep any noise to a minimum, with the IC and Q10/Q11 forming a feedback loop do as the volume is decreased so is the gain. The volume control is a 32 step attenuator with differing attenuation steps ranging from smaller to larger as the volume is increased. As I understand it, the wiper of the volume pot is held at zero volts by the IC, has no signal on it at all. Consequently the replacement of the original TL072 is the modern lower noise TL2072 as the IC is not handling audio.

I'm aware of how the volume circuit works.

I may have misunderstood your post intention. I presumed it to mean that the volume TL072 should NOT be replaced with anything other than the updated TLE2072 (for some technical reason).

If I understand now, you're NOT saying that it SHOULD not be replaced, just that there's little to be gained by doing so?

 

[pwauk12]

@HalfApt I always replace the 072 with a 2072- did not mean to "mansplane" the circuit

@mcgjohn - The early versions had a sealed transformer which was tiny and does fail. If the unit has that transformer, I replace it with a Triad 700ma unit, which keeps things in the same footprint. There is also a torodial that will fit. I have considered an outboard supply, though keeping the original external looks has been important, as has the switched AC for the APT power amps on the rear. I believe the secondary is 18V.

I have wondered what a fully regulated supply using LM series regulators with a larger transformer and a substantial set of power supply caps would do.

 

[GKTAUDIO]

I have found 3 different transformers in stock Apt preamps. As I recall, two models had 18V secondaries, and one model had a single 15V secondary.

Here is a thread I posted about my transformer replacements:
https://audiokarma.org/forums/index...er-replacement-projects.954371/#post-15146272

Since that time, I have experimented with an outboard power supply using a 130VA toroidal, as well as a tightly regulated bench lab power supply. I don't have a spectrum analyzer, but do have a Keysight signal generator and various Tek 'scopes, and was not able to notice any change in signal output waveform with these more massive power supplies, nor did I hear any changes. I think there is a point of diminishing returns; the current draw at signal peaks and troughs is just not so high that a relatively huge PS is needed. I've found this to be true with Threshold preamps for which I've made larger power supplies.

On the other hand, I have noticed a change in sound quality with the two transformer replacements I made previously in the Apt preamps - a toroidal shoehorned into the chassis, and a much better-fitting, slightly larger, higher VA EI transformer that required little modification (thanks to HalfApt for pointing me to that one). Keep in mind that all of these changes were made to units that had most or all op amps replaced with more power-hungry devices - certainly what caused failure in the second unit I described.

 

[HalfApt]

Besides all the opamps, there are two main weak points on the apt preamp.

1. the tiny 12 watt power supply.
2. all the interstage electrolytic coupling caps used in the audio signal path.

1. Power budget is rarely considered when "rolling" opamps.

I'm sure T.Holman designed the power supply to adequately cover the worst case draws from TL072s plus some reserve, but did not anticipate significantly more.

IMHO the OPA2134 is a poor choice in that regard, as it consumes nearly 3x more current than the original TL072, and may push the power supply to, or over, its capability.

Ranked in increasing power requirement (per amp, 2 amps per chip):

• TL072: 1.4mA nom, 2.5mA max
• TLE2072: 1.7mA nom, 2.2mA max
• OPA1642: 1.8mA nom., 2.3mA max
• OPA2134: 4mA nom., 5mA max

The OPA1642 is said to be an "improved" 2134, with some better specs, and significantly less power requirement. Unfortunately, it's SOIC only, so not as DIY friendly, but that's what I used and they work great with the original sealed transformer.

Another issue rarely considered is power pin bypass caps. The Apt has a number of them, "good enough" for the TL072, but lacking for the more "high strung" opamps. More should be added when "rolling".

2. The electrolytic coupling caps CAN be entirely replaced with poly, if one is willing to put in the time, effort & money.

 

[GKTAUDIO]

IMHO the OPA2134 is a poor choice in that regard, as it consumes nearly 3x more current than the original TL072, and may push the power supply to, or over, its capability.

Yes, I concur. I had to bump up the fuse rating on my 3 modified preamps with all op amps replaced. The cost was quite high for that many op amps, so I've been loathe to replace them, but in future may do so, and it would be with the OPA1642 as you have done - I have plenty of SOIC adapater boards.

 

[pwauk12]

and thanks to @gtkaudio for helping me sort out the first transformer failure.

I do beef up the power supply when installing socketed 2134's, larger power caps and some additional decoupling at a couple of circuit locations seems to work well.

I use mainly Nichicon MUSE UES BP 22uf for the audio, which I find works well and gives suitably low distortion and low end response, except in couple of places where I use polypropylene film.

I love what @HalfApt has done- simply beautiful and functional work!

on the power budget issue, an eye opener was the power supply on the Kenwood L08C preamp, which was I thought over engineered, until measured and listened to.