JAY X

Fader + trafo buffer
« on: November 21, 2019, 11:50:18 AM »
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



ruffrecords

Re: Fader + trafo buffer
« Reply #1 on: November 21, 2019, 12:28:32 PM »
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
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

JAY X

Re: Fader + trafo buffer
« Reply #2 on: November 21, 2019, 01:37:52 PM »
Hi Ian!,

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

Jay x

ruffrecords

Re: Fader + trafo buffer
« Reply #3 on: November 21, 2019, 03:13:51 PM »
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
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

JAY X

Re: Fader + trafo buffer
« Reply #4 on: November 25, 2019, 05:55:06 AM »
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

ruffrecords

Re: Fader + trafo buffer
« Reply #5 on: November 25, 2019, 12:45:42 PM »
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.
Quote
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
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

JAY X

Re: Fader + trafo buffer
« Reply #6 on: November 26, 2019, 03:49:21 AM »
Hi Ian!

 
Quote
The BA 283 would replace the fader amplifier.

Ok, Great!!

Jay x

« Last Edit: November 26, 2019, 08:23:38 AM by JAY X »

JAY X

Re: Fader + trafo buffer
« Reply #7 on: November 28, 2019, 05:27:10 AM »
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

ruffrecords

Re: Fader + trafo buffer
« Reply #8 on: November 28, 2019, 12:38:31 PM »
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
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

abbey road d enfer

Re: Fader + trafo buffer
« Reply #9 on: December 06, 2019, 01:36:13 PM »
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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


Re: Fader + trafo buffer
« Reply #10 on: December 07, 2019, 06:26:40 AM »
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.


JAY X

Re: Fader + trafo buffer
« Reply #11 on: December 09, 2019, 10:39:23 AM »
Hi!

Ok, i will try to post a new schematic with these suggestions!

Jay x


 

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