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Output section advice
Old 2nd June 2020
  #1
Output section advice

Hey
I’m tweaking out a urei562. I’m looking at the output section and want to add a pot to control the output drive. Any advice? I’m thinking it’s rn-8a Just before the op amp.

Can anyone please explain to me how this output section works? It’s an opamp that drives a pair of transistors that look like a push-pull tube amp output that drives the output transformer.

Thanks!!
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Old 2nd June 2020
  #2
Lives for gear
It's a simple class AB output stage as seen often driving output transformers and headphones. The diodes are the bias spreader, they are biased by the 4k7 resistor and ensure that there are two Vbe drops between the bases of the transistors. Due to the high-ish value emitter resistors (required for bias stabilization) it's probably underbiased, so feedback does what it can about the resulting crossover distortion. Not that with 741 opamps you would expect low distortion from the circuit anyway.

As it is the gain is 1 + 680 / 3k9 = 1.17. You can indeed make the gain adjustable by replacing rn-8a with a pot in series with a resistor that will set the maximum gain (e.g. 68R so the max. gain is 11). Ideally you want a reverse log pot there of say 5k. Note that at higher gains the transistors may run out of steam, which are they?
Old 23rd July 2020
  #3
Quote:
Originally Posted by Cabirio View Post
It's a simple class AB output stage as seen often driving output transformers and headphones. The diodes are the bias spreader, they are biased by the 4k7 resistor and ensure that there are two Vbe drops between the bases of the transistors. Due to the high-ish value emitter resistors (required for bias stabilization) it's probably underbiased, so feedback does what it can about the resulting crossover distortion. Not that with 741 opamps you would expect low distortion from the circuit anyway.

As it is the gain is 1 + 680 / 3k9 = 1.17. You can indeed make the gain adjustable by replacing rn-8a with a pot in series with a resistor that will set the maximum gain (e.g. 68R so the max. gain is 11). Ideally you want a reverse log pot there of say 5k. Note that at higher gains the transistors may run out of steam, which are they?
Sorry Cabirio, I got swamped with work for 2 months and this project sat in the corner, but not forgotten!

I appreciate the advice. I will try to replace rn8a with a pot, although it might be tough, it’s a 16pin chip with 8 resistors on it. Maybe I can lift the legs, or just replace the whole thing with 7 resistors and a pot. I’ll check and see what I have in my parts bin.

The transistors are:
Q1 952 2n6716
Q2 018 2n6728

The opamps are all actually TL074. I think 2074 can replace them with better sonics.
Old 23rd July 2020
  #4
Lives for gear
Quote:
Originally Posted by enginefire View Post
The transistors are:
Q1 952 2n6716
Q2 018 2n6728

The opamps are all actually TL074. I think 2074 can replace them with better sonics.
The transistors will be fine, and yes, the TLE2074 (I guess that's the one you mean) is better than the TL074 in pretty much every respect and can take +/-18 V rails, so why not, though I wouldn't expect spectacularly audible results.

Good luck with the resistor surgery and let us know how it works out.
Old 31st July 2020
  #5
Quote:
Originally Posted by Cabirio View Post
It's a simple class AB output stage as seen often driving output transformers and headphones. The diodes are the bias spreader, they are biased by the 4k7 resistor and ensure that there are two Vbe drops between the bases of the transistors. Due to the high-ish value emitter resistors (required for bias stabilization) it's probably underbiased, so feedback does what it can about the resulting crossover distortion. Not that with 741 opamps you would expect low distortion from the circuit anyway.

As it is the gain is 1 + 680 / 3k9 = 1.17. You can indeed make the gain adjustable by replacing rn-8a with a pot in series with a resistor that will set the maximum gain (e.g. 68R so the max. gain is 11). Ideally you want a reverse log pot there of say 5k. Note that at higher gains the transistors may run out of steam, which are they?
I added a pot on the output, but I can’t seem to increase the gain much beyond its stock amount without getting a lot of overdrive. Sounds kinda cool but it’s not the outcome I was hoping for.... any other suggestions ? Maybe change up the transistors?
Old 31st July 2020
  #6
Lives for gear
It's probably saturating the output transformer and/or hitting the rails, i.e. clipping, what would happen first depends on the transformer. With 18 V rails the maximum amplitude at the transformer input before the output stage clips will be about +23 dBu. Note also that you may well be saturating the input stage of whatever the Urei is connected to, you may want to put a voltage divider / pot at the output to make sure that's not what's happening.
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