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Re: Cohen/Transamp output stage

Posted: Wed Dec 30, 2020 4:17 pm
by mediatechnology
Gold wrote:
Wed Dec 30, 2020 3:39 pm
JR. wrote:
Tue Dec 29, 2020 2:06 pm
If you want to have some fun, do some null testing between your favorite preamps and each other or perhaps even the reference input with preamp wiring inverting.
A null test won’t tell you anything about the dynamic behavior of the transformer. Without quantifying the dynamic behavior you get an incomplete picture of what the preamp is doing in the wild.
Null test with music.

I would null test with music after removing Raytheon RC5534 op amps from SSLs and replacing them with Signetics NE5534s.
A pair of RC5532-equipped channels wouldn't null well with each other and the residual was pretty gritty.
A pair of Signetics 5532 channels would null well and the residual sounded like the attenuated source.
It was pretty convincing for the client to hear the difference.
I couldn't have done this running tone.

I replaced thousands of RC5532s at no charge to the client.

Re: Cohen/Transamp output stage

Posted: Wed Dec 30, 2020 4:36 pm
by ilya
So to make sure I did that correctly, here's the crude scheme attached. Is it how 1606 should be connected to the Cohen circuit?

Re: Cohen/Transamp output stage

Posted: Wed Dec 30, 2020 4:44 pm
by mediatechnology
ilya wrote:
Wed Dec 30, 2020 4:36 pm
So to make sure I did that correctly, here's the crude scheme attached. Is it how 1606 should be connected to the Cohen circuit?
That's it though the polarities on the 1606 are reversed.
The top one is 1606 positive.

Re: Cohen/Transamp output stage

Posted: Wed Dec 30, 2020 4:49 pm
by ilya
mediatechnology wrote:
Wed Dec 30, 2020 4:44 pm
That's it though the polarities on the 1606 are reversed.
The top one is 1606 positive.
Well, of course! For some reason I thought those were inverting stages. Doh!
Thanks for spotting this!

Re: Cohen/Transamp output stage

Posted: Wed Dec 30, 2020 4:51 pm
by mediatechnology
The discrete stage inverts and the op amp which follows it inverts it again.
So its non-inverting overall.

Re: Cohen/Transamp output stage

Posted: Wed Dec 30, 2020 5:18 pm
by Gold
mediatechnology wrote:
Wed Dec 30, 2020 4:17 pm
Null test with music.
Good Idea!

Re: Cohen/Transamp output stage

Posted: Thu Dec 31, 2020 10:26 am
by JR.
Gold wrote:
Wed Dec 30, 2020 5:18 pm
mediatechnology wrote:
Wed Dec 30, 2020 4:17 pm
Null test with music.
Good Idea!
I used null testing a lot back last century, not so much lately.

Great for comparing near identical paths, like two console strips, while subtle innocuous path differences can dramatically diminish the null product depth. For revealing sound character of low level intermodulation distortions normally masked by audio and noise floor (like Wayne's example) this can be very revealing.

That said minor frequency response differences will show up in nulls, even inaudible phase shift will diminish null depth. Also look out for simple path delay between digital audio stems.

One final caveat, null testing only reveals that two stems are different, not which is which.

JR

Re: Cohen/Transamp output stage

Posted: Fri Jan 01, 2021 1:25 pm
by Gold
JR. wrote:
Thu Dec 31, 2020 10:26 am
One final caveat, null testing only reveals that two stems are different, not which is which.
In the case of a transformer input microphone preamp vs transformerless pre the one that has artifacts might be preferable in use.

Re: Cohen/Transamp output stage

Posted: Sat Jan 02, 2021 8:40 am
by ilya
Very interesting discussion!
While we're wandering around the subject, I've got one more question. In TA documents, Valley recommend to use tantalums as phantom blocking caps. What's the reason behind this? Why tantalums?

Re: Cohen/Transamp output stage

Posted: Sat Jan 02, 2021 10:05 am
by JR.
ilya wrote:
Sat Jan 02, 2021 8:40 am
Very interesting discussion!
While we're wandering around the subject, I've got one more question. In TA documents, Valley recommend to use tantalums as phantom blocking caps. What's the reason behind this? Why tantalums?
Old advice.... Tantalum caps were generally lower ESR than aluminum caps of the time. Modern high quality aluminum should be fine, or no capcitors like Wayne's design.

JR