Adding remote control to a Dynaco PAS3

[kotofei]

With 100k pot you are dangerously close "as long as you don't connect anything to the tape out jacks"
It's certainly not a general preamp with built-in margins and a usable tape-out connector.
I also used a 100k pot BUT i introduced a cathode follower stage before the (same) pot, thus keeping
the full potential of the amp.

Yes, tape-out jack loading is another problem and buffered of this output is in plans. I'm not going to use tape out for now.

I believe I saw a post of yours which briefly described an opamp-based PAS3 output buffer. How did you power it, specifically, where did you tap AC? If buffer requires +/- 16V DC, I might need to get a power transformer with two secondaries: for remote control and for the buffer.

 

[peterh]

Yes, tape-out jack loading is another problem and buffered of this output is in plans. I'm not going to use tape out for now.

I believe I saw a post of yours which briefly described an opamp-based PAS3 output buffer. How did you power it, specifically, where did you tap AC? If buffer requires +/- 16V DC, I might need to get a power transformer with two secondaries: for remote control and for the buffer.

It was a cmos-buffer i found ( in canada i think), and no i did not use it. Instead i build the buffers and
remote volume control in a separate chassis ( the fm1 mentioned at the beginning of this thread
http://n.manet.nu/fm1/fm1.html )

Whatever buffer except the cmos buffer i saw ( not op-amp based) needs an additional power supply, just like
power to the remote. Adding both buffers and remote in an external box solved all this. And my buffer is
silicon-free in the signal path

 

[powerNtubes]

Please pardon my ignorance but how did you measured the current with the scope? I planned to do this with my DMM set as ammeter.

My portable service scope has a DMM built it......sorry for the confusion.

 

[gadget73]

using the scope to get a voltage reading across a resistor and some math would do it too, its just not nearly as convenient as using a current meter

 

[audioregen]

A little more on the 100K volume pot installation:

Replacing the stock 250K volume pot. with 100K will,essentially, have two effects. First it, will reduce the sensitivity of the line stage by about 1.8dB. This is due to the fact that the 47K series resistor, located on the blend selector switch, forms a voltage divider with the volume and balance pots. So, the volume would be set just a little bit higher to achieve the same level at the output. But also, the 100K pot will somewhat degrade the low end response of the phono stage due to the additional loading. Assuming nothing is connected to the tape out jack, the response will be down about -1.3dB at 20Hz, which may not be audible.

Should you deem it necessary, the low end response can be restored by simply changing the "second" output coupling cap. in the phono stage from 0.1uF to 0.22uF. This will also restore the stock load handling capability at the tape out jack. Should you wish to drive lower impedance SS loads from the tape out jack (and it seems like you do) then a buffer of some type will be necessary. Note - the coupling capacitor increase applies only to the 100K volume pot condition, doing so with a stock pot will only serve to degrade the RIAA response.

 

[kotofei]

A little more on the 100K volume pot installation:

Replacing the stock 250K volume pot. with 100K will,essentially, have two effects. First it, will reduce the sensitivity of the line stage by about 1.8dB. This is due to the fact that the 47K series resistor, located on the blend selector switch, forms a voltage divider with the volume and balance pots. So, the volume would be set just a little bit higher to achieve the same level at the output. But also, the 100K pot will somewhat degrade the low end response of the phono stage due to the additional loading. Assuming nothing is connected to the tape out jack, the response will be down about -1.3dB at 20Hz, which may not be audible.

Should you deem it necessary, the low end response can be restored by simply changing the "second" output coupling cap. in the phono stage from 0.1uF to 0.22uF. This will also restore the stock load handling capability at the tape out jack. Should you wish to drive lower impedance SS loads from the tape out jack (and it seems like you do) then a buffer of some type will be necessary. Note - the coupling capacitor increase applies only to the 100K volume pot condition, doing so with a stock pot will only serve to degrade the RIAA response.

Thanks a lot for the great insight!

Actually I wanted to eliminate the blend selector with the associated 47 K resistor and the 0.1 mFd cap.

Second output coupling cap in the line stage is the one that goes to Pin 2 of the SSA/SSC selector, NOT the one connected to the plate of the second triode, right? I'll definitively change it together with the 4.7M resistor in the feedback RIAA loop that you recommended to replace with 3.9 M.

The other option as per your PAS Phono stage analysis article will be to implement "Dynaco Doctor" modifications with further doctoring them by replacing the 2M resistor with 3.3M. Perhaps this allow to correct the -0.5 dB deviation at 200 Hz?

 

[audioregen]

Thanks a lot for the great insight!

Actually I wanted to eliminate the blend selector with the associated 47 K resistor and the 0.1 mFd cap.

Second output coupling cap in the line stage is the one that goes to Pin 2 of the SSA/SSC selector, NOT the one connected to the plate of the second triode, right? I'll definitively change it together with the 4.7M resistor in the feedback RIAA loop that you recommended to replace with 3.9 M.

The other option as per your PAS Phono stage analysis article will be to implement "Dynaco Doctor" modifications with further doctoring them by replacing the 2M resistor with 3.3M. Perhaps this allow to correct the -0.5 dB deviation at 200 Hz?

It's important to note that the 47K resistors on the blend switch are, in effect, in series with the parallel combination of the volume and balance pots. Thus, they are an integral and important part of the load that the phono stage "sees." If you delete them, you will effectively reduce the load impedance by an additional 47K, in addition to the already lower impedance incurred by the installation of the 100K volume pot. Now, the increased load burden becomes very significant, and all the other considerations and suggestions mentioned here, or in my article, no longer apply since they are based on the stock circuit with the blend functions intact. Of course, that doesn't mean something couldn't be worked out, only that I haven't considered such a condition thus far. You might want to consider approaching this project in stages. First, just concentrate on your main objective, which would seem to be remote volume control, and the issues related to that (power supply, etc). Once you have that sorted out and working, then move on to modifications like improving the RIAA response with the 100K pot, and bypassing the blend switch (do you really need to?). The reason the 47K resistors exist is to serve as source isolation resistors when the blend features are employed. It's just a simple resitive mixer really, which is out of circuit when set to the full stereo position, except for the 47K resistors which, in effect, are always in series with the line stage. Unless you need the panel space for you remote volume control, maybe you could just leave it?

As for the .1uF capacitors, you are correct. NOT the one connected to the plate. It is common to assume that these are "just" coupling capacitors. Although they serve that purpose, obviously, the importance of their interaction in controlling the low end of the RIAA response is not immediately evident to many. It's not a matter of "just make it large enough to pass the lowest frequency of interest." The specific reactance they present plays an important role, and Ed Laurent had this pretty well figured out to optimize the circuit, as I explained in my article. Thus, when I recommended 0.22uF above, that is the closest common value that would result in the response being close to the stock circuit, given the load present with the 100K pot. Making it any larger would only result in an elevated peak in the low end response. The capacitor on the plate is, effectively, within the FB loop, but still does play a role, just note quite as much as the second cap. And so, it's all a matter of what the actual "load" really ends up being. The capacitors are adjusted to suite. Of course, there are limits to what that load can be.

 

[gadget73]

Thanks for the info re the .22 caps. I have a 100k pot in mine and i recently un-did the "fixes" to the RIAA stage. I put the 0.1 caps back in. I'll swap them for some 0.22 if that makes it play nicer with the 100k pot. I had them out previously as per the "fix" article but it made this god-awful thump when switching in and out of phono mode thanks to the DC voltage coming through the switch that wouldn't be there otherwise.

 

[kotofei]

I got a power transformer for the project, it's a Philmore TR030, output 6V 500 mA. Got it from Fry's for approximately $5 with free shipping.

The tranny will be installed either at the left of filament filtering caps ( in fact, I ordered SDS power supply plate that will be mounted where caps are now) or instead of the can cap since it's no longer needed if SDS plate installed.

The remote control assembly hopefully should arrive in the next couple of days.

 

[kotofei]

The kit arrived, without input selector switch. Shouldn't be a big deal (hopefully); seller already shipped it.

Seems like there is enough space for the motorized pot to be installed instead of the stock volume control. Can't say for sure since one of the previous owners broke the screw that holds the knob on the volume control shaft. I'll saw the shaft off between chassis and the faceplate with the thin blade.

Placement of control board with infrared sensor is more problematic. The only space at the front is where tone controls are; I wanted to bypass them anyways but now I'll need to remove pots and leave holes at the faceplate. On the other hand, one of these holes could be used for housing of infrared sensor.

.

 

[gadget73]

If you aren't upset about losing the pilot lamp, you could extend the wiring to the IR sensor and put it in that location. Looks like about the right size.

Alternatively, maybe lose one of the switches on the front. The scratch filter is useless. A missing switch would be less odd looking than missing controls I think.

 

[kotofei]

...
Alternatively, maybe lose one of the switches on the front. The scratch filter is useless. A missing switch would be less odd looking than missing controls I think.

That's good idea; scratch filter will be removed at some point. However, the space occupied now by the tone control pots looks like the only place for mounting of the remote board. I might try to mount it using the long "standoffs" ABOVE the pots... Need to think it over.

 

[gadget73]

I certainly wouldn't dissuade you from pulling the tone controls, just thinking that cosmetically it might look odd with one missing for the IR eye to poke through, or the whole bank of them gone and a set of holes left to fill. If they need to go away entirely, people do make replacement faceplates without the tone controls present. I think most of those also lack the switches across the bottom, since those are also commonly disconnected on things with the tone controls pulled.

 

[BillWojo]

A friend of mine went through the IR sensor thing on another preamp. He was at the flea market and found some sort of remote IR sensor EYE on a long cable with a plug on it. It was part of a Comcast box and the sensor could be placed remotely, had I think a 3 foot cord. He installed a jack on the back of the preamp and it worked great. If I ever do a remote on my PAS3X, I will use that system.

BillWojo

 

[kotofei]

A friend of mine went through the IR sensor thing on another preamp. He was at the flea market and found some sort of remote IR sensor EYE on a long cable with a plug on it. It was part of a Comcast box and the sensor could be placed remotely, had I think a 3 foot cord. He installed a jack on the back of the preamp and it worked great. If I ever do a remote on my PAS3X, I will use that system.

BillWojo

I saw a few IR sensors with cables on this auction site. However, rather than purchasing one, I can unsolder an existing IR sensor from the board and mount it in the scratch filter hole running a cable to the board.

 

[Lavane]

I certainly wouldn't dissuade you from pulling the tone controls, just thinking that cosmetically it might look odd with one missing for the IR eye to poke through, or the whole bank of them gone and a set of holes left to fill. If they need to go away entirely, people do make replacement faceplates without the tone controls present. I think most of those also lack the switches across the bottom, since those are also commonly disconnected on things with the tone controls pulled.

I think Dynakit sells a silver and a black face plate without switch or tone control holes.

 

[gadget73]

yeah, maybe thats where I saw it. I know I've seen them though.

 

[kotofei]

I evaluated different options how to fit all components inside the Dynaco chassis and the plan now is:

Replace the original volume pot with the motorized pot from the kit;
Remove the scratch filter and switch; this creates an opening for mounting IR EYE;
Mount the main remote control board on stands behind tone controls, leaving pots in place;
Mount the remote transformer where the Dynaco cap can now is;
Replace the cap can and original filament caps and diodes with SDS Power supply board;
Replace stock Input selector with relays ( this will be Stage II).

So I started with removing the old volume pot, then made a 3 mm hole for the "tooth" it has for positioning. Pot fits well but the shaft was too long and I sized it.

Now I can't quite decide what is the best way to connect the new pot. The old one had four "legs" : the one at the top for loudness and three at the bottom. The new pot doesn't have loudness tap so its components, 0.1 uF cap and the 47 KOhm resistor will be gone ( I am not 100% sure about resistor since it remains in circuit in the loudness OFF position according to schematics but in my Dynaco it's disconnected from circuit in the OFF position).

The three other legs go to line amp input ( pins #2 and #9), to taper of the balance pot and to ground (?). It looks like ground, so should I connect "ground" of the new pot to it?

One other thing I don't quite understand is the crosstalk between two channels of the new pot.Specifically, input of channel A shows 95-190 KOhm resistance to the output of the Channel B and this resistance changes with the pot rotation. Should it be like this?

 

[Hi-Q]

This looks like a good starter kit with many more outputs available. One has to add extra latches or maybe program a PIC to do the relay/audio control selection.
https://www.quasarelectronics.co.uk/Item/3092-14-ch-infrared-remote-control-decoder-ic-pack

 

[kotofei]

UPDATE: First stage done. Motorized volume pot, controller plate and small power transformer powering them installed and everything seems working. Took me a lot of time just to free up space for the new components.