Fader + trafo buffer

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JAY X

Well-known member
Joined
Jan 9, 2009
Messages
683
Hi!

Attached is a schematic of a fader amplifier with an inductor (L32)  in paralell to the output resistor.(R145).

I thought that instead of using another opamp as buffer for the output transformer,(Sowter 8403X), i could combine both circuits within the fader amplifier. But i'm not sure...This is why i ask here... :D

Thank you for your help!
Jay x

 

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  • fader and trafo buffer.jpg
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It is not clear to me what is wrong with the circuit as it stands. Is it the inductor that bothers you. It is only 3.9 microhenries so it is very small. It is necessary for op amp stability.

Cheers

Ian
 
Hi Ian!,

So, the circuit is correct.
I just wanted to make sure it is ok. :)

Jay x
 
JAY X said:
Hi Ian!,

So, the circuit is correct.
I just wanted to make sure it is ok. :)

Jay x

Yes, it is correct. The series resistor with a parallel inductor is common in transformer drivers.

Cheers

Ian
 
Hi!

In case i change the type/brand of output transformer (carnhill CA 18 2407, or VTB 2407), with  the same circuit, i have to change the inductor value acordingly..¿right?

Another different story would be to drive output transformers with a class A circuit like the BA283AM...in that case i don't need the inductor, but follow the fader amplifier with the BA283 output stage... 8)

Jay x
 
JAY X said:
Hi!

In case i change the type/brand of output transformer (carnhill CA 18 2407, or VTB 2407), with  the same circuit, i have to change the inductor value acordingly..¿right?
Pass. I am not familiar enough with that design to be able to answer sensibly.
Another different story would be to drive output transformers with a class A circuit like the BA283AM...in that case i don't need the inductor, but follow the fader amplifier with the BA283 output stage... 8)

Jay x

The BA 283 would replace the fader amplifier.

Cheers

Ian
 
Hi!

After reading more about Neve circuitry, I see that some parts run at 24v and others at +/-12v. (BA512 opamps).
Running these circuits at higher voltages, ( +/-15v or +/- 18v) would require change some bias resistors.
But as many audio opamps like NE5532 can work at +/- 12v, it is no problem combining these stages together, except for voltage swing.

Jay x
 
When I was at Neve back in the mid 70s, everything ran on 24V. Audio circuits, relays and lamps all from one 24V power supply. While I was there, the R&D department carried out a range of tests on a brand new op amp that had just been released by Philips. It was not called the NE5534 at that time. Instead it had a TDA number (I cannot rememeber the number) but it later became the NE5534 and NE5532. They were not introduced into Neve products until after I left  so I do not know to what degree the two types of circuit lived alongside each other.

Cheers

Ian
 
JAY X said:
So, the circuit is correct.
I just wanted to make sure it is ok. :)
It is correct in the sense that it will make some sound, but you don't need the inductor, the 330pF cap is largely enough to ensure stability.
I guess this LR network was intende to decouple capacitive loads, which is not necessary anymore since you have a xfmr.
I would ditch the inductor and the 47r, increase the output cap, since it sees very small offset voltage, you could use a 6V rated cap and increase it to 1000uF.
I would also implement two paths for negative feedback, one as it is but increasing R146 to 10k and adding a 10k from the other side of C74; that would be in order to tame the VLF hump due to the resonance of the output cap with the primary of the xfmr.
 
abbey road d enfer said:
It is correct in the sense that it will make some sound, but you don't need the inductor, the 330pF cap is largely enough to ensure stability.
I guess this LR network was intende to decouple capacitive loads, which is not necessary anymore since you have a xfmr.
I would ditch the inductor and the 47r, increase the output cap, since it sees very small offset voltage, you could use a 6V rated cap and increase it to 1000uF.
I would also implement two paths for negative feedback, one as it is but increasing R146 to 10k and adding a 10k from the other side of C74; that would be in order to tame the VLF hump due to the resonance of the output cap with the primary of the xfmr.

All good suggestions and advice.

 
Hi!

I know this thread is a bit old, and I'm a bit slow posting... but Ii need to know if the schematic is ok. It is an inverting fader amplifier followed by an inverting mixed feedback driver. As both stages are inverting, in theory a signal coming in phase in the first stage should be in phase at the balanced output...¿Right?. but I'm not completely sure...🤔

Note 1: The mixed feedback driver schematic is from analog devices opamp applications handbook, chapter 6 figure 6-61. section 6.72.

Note 2: LL2811 Primary pin1 is + phase, pin6 is - phase. pins 3&8 are strapped.
LL2811 Secondary pin 9 is + phase. pin 14 is - phase. pins 11-16 are strapped.
 

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Last edited:
Hi!

I know this thread is a bit old, and I'm a bit slow posting... but Ii need to know if the schematic is ok. It is an inverting fader amplifier followed by an inverting mixed feedback driver. As both stages are inverting, in theory a signal coming in phase in the first stage should be in phase at the balanced output...
Correct.
 
Hi Abbey!

Ok!, I was doubting a lot...😅. Now the last question, if i add a signal derivation from the +phase transformer primary to a line driver like the that1646, (G=2), will I have the same voltage output as in the secondary winding of the transformer?
 
Now the last question, if i add a signal derivation from the +phase transformer primary to a line driver like the that1646, (G=2), will I have the same voltage output as in the secondary winding of the transformer?
I don't think so. The xfmr is a 1:1, right? So the output voltage is the same as the primary voltage. The output voltage from the 1646 would be 6dB higher.
Anyway you should know that the signal that drives the xfmr is pre-distorted in a way that's opposite to the xfmr distortion, so it's not a clean signal.
You should add a separate path from the fader's wiper, with 6dB attenuation and a buffer to provide low-Z drive the 1646.
 
Hi!

Yes the trafo is wired as 1:1. (2:2).

So, to understand better, the output of the transformer driver is not suitable to drive an attenuator and the 1646, because of the special configuration of the transformer driver, that generates a "pre-distorted" signal to counter- act the transformer distortion...
 
Hi!

Attached are two drawings for the LL2811 transformer. Both wired as 2:2 (1:1). But the wiring order is different. The first left arrangement is used with balanced driving, and the right arrangement with unbalanced driving. ¿are they exchangeable?

Jay x
 

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Hi!

Attached are two drawings for the LL2811 transformer. Both wired as 2:2 (1:1). But the wiring order is different.
The difference is that both the primary and the secondary are polarity-reversed, for a net identical result.*
The first left arrangement is used with balanced driving, and the right arrangement with unbalanced driving. ¿are they exchangeable?
Yes.

* Unless you consider the possible effects of unbalanced stray capacitances.
 
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