34: Replacing R3/R8 with symmetrical power rails

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30-Oct-2019 02:12 PM

Fabian

Fabian

Posts: 102

It has been suggested occasionally on the forum to change the stock power rails (-9.4V and +8.6V) to symmetrical values (+/- 9V) when using modern op-amps in the 34. Regardless of if there is much to gain from this mod I decided to give it a try.

However, I just noticed that the power supply to the phono input modules is asymmetric as well, so the voltage drop across R3 and R8 is roughly the same when the power rails are in stock condition. I did not find any reference to R3/8 in any of the relevant topics, but from my understanding R3 and R8 should be adjusted as well, when making the supply rails symmetrical.

For the same voltage drop I would choose 620Ohm resistors.

Right or wrong? 

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30-Oct-2019 03:21 PM

oertbling

oertbling

Posts: 1

The asymetrical powerlines have a biasing function for the electrolytic capacitors in the signal path. Without this bias they will disfunction in due time. 

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31-Oct-2019 02:09 AM

EJP

EJP

Posts: 1669

Well spotted. 620R is OK. This modification makes sense once you get rid of all the TL071/2 opamps in the audio path (don't change the one in the PSU), but I can never be bothered to actually do it.

 

The stuff about the asymmetrical rails having a biasing function for the coupling electrolytics is urban myth. This is impossible. The voltage at the output of the opamps is zero give or take a few mV, as determined by the NFB around them and one input being referenced to ground. It is not determined by a small difference between the voltage rails.

 

The purpose of the asymmetry is to ensure that the TL071s' output hits the positive rail before the negative rail, as they behave very badly when they hit the negative rail, latching up positive. In practice it can't hit either rail anyway because of the protection diodes on the inputs.

 

EJP

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31-Oct-2019 03:54 PM

Fabian

Fabian

Posts: 102

Thanks for confirmation, EJP.

The myth probably goes back mainly to Keith Snook, but your explanation makes a lot more sense to me.

Of course, I am using LME49710/LM4562 as supplied in the kit, not the old TL071/TL072. I also replaced all coupling caps with wire links (except in the tape output, where I used non-polar Nichicon Muse ES for now).

I'm also replacing (almost) all the carbon film with metal film resistors (hey, I'm doing this for fun, not for business). So there is no extra effort in making the supply voltages symmetric.

Should there be a difference in sound over my other 34, I will not know the reason for sure. The other unit did not get the MAX4066 switches.

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