Original UA console - 100D preamp, EQ, 101D program amp

emrr · Jan 18, 2011

A PDF with descriptives can be found in the group Gmail account. These are screen captures from that PDF. NOT the 610, this console design would be responsible for many of the hits for which the 610 has been given credit. I have once seen a few of the 100D modules show up at ebay. Note the EQ is referred to by two different model numbers. The 100D looks a lot like the later 108. Debate and discuss.

lassoharp · Jan 18, 2011

There seems to be a lot going on in both amps. First cascode input stage on a mic amp I've ever seen.

Q: How does C10 (OT CT to cathode loop) aid in insuring the EQ will work properly?

Q: What would be the effects of removing C4 in the program amp? Is it curing a hi freq roll off or rising response? Why is it on one side only of a PP amp?

ruffrecords · Jan 18, 2011

The 610 seems to me to be a 'cost reduced' version of the 100 series. The 610 has both tubes set up as two stage CC with NFB which allows the channel fader to be placed between the two stages. The limited 610 EQ looks like a very cut down version of the 100 series implemented in the NFB of the second tube and because there is only one tube driving the output transformer it has a 30K primary which throws away 17dB of gain and may go some way to explaining anecdotal reports of distortion in the 610 under some conditions.

Cheers

Ian

emrr · Jan 18, 2011

C7 in the 101D is a touch I don't remember seeing before either.

ruffrecords · Jan 18, 2011

lassoharp said:
Q: How does C10 (OT CT to cathode loop) aid in insuring the EQ will work properly?

It is there to ensure NFB stability. It is only 450 pico Farads and therefore has no effect on the EQ

Q: What would be the effects of removing C4 in the program amp? Is it curing a hi freq roll off or rising response? Why is it on one side only of a PP amp?

It is there for the same reason, NFB stability. V2 almost certainly runs in class A so both anodes see the full cycle of the waveform so you only need use one of them.

P.S How would I access the group Gmail account???

Cheers

Ian

warpie · Jan 18, 2011

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emrr · Jan 18, 2011

ruffrecords said:
lassoharp said:

Q: How does C10 (OT CT to cathode loop) aid in insuring the EQ will work properly?

Click to expand...

It is there to ensure NFB stability. It is only 450 pico Farads and therefore has no effect on the EQ

Interesting point to pull from. Convenience of a transformer CT they used, or a special design? Would guess an existing transformer CT, and saves use of one resistor. Or, point picked to simplify parts count and account for some output transformer effects, while still isolated from tertiary winding and possible EQ network.

ruffrecords · Jan 18, 2011

emrr said:
ruffrecords said:

lassoharp said:

Q: How does C10 (OT CT to cathode loop) aid in insuring the EQ will work properly?

Click to expand...

It is there to ensure NFB stability. It is only 450 pico Farads and therefore has no effect on the EQ

Click to expand...

Interesting point to pull from. Convenience of a transformer CT they used, or a special design? Would guess an existing transformer CT, and saves use of one resistor. Or, point picked to simplify parts count and account for some output transformer effects, while still isolated from tertiary winding and possible EQ network.

Difficult to tell if the transformer tap is at the centre or not, but if it was an existing transformer with a suitable CT then why not use it. The advantange is that at least some of the transformer characteristic is included in the loop containing the NFB stability cap.

Cheers

Ian

bockaudio · Jan 18, 2011

VERY interesting.
BTW, "some reports of distortion in the 610" is borderline silly, it's a fuzzbox!

ruffrecords · Jan 18, 2011

bockaudio said:
VERY interesting.
BTW, "some reports of distortion in the 610" is borderline silly, it's a fuzzbox!

I only mentioned it because of recently reading 'Temples of Sound' where one engineer claimed that 610 modules distorted on mix down but were OK for direct to tape. The most likely reason for this that the 610 line input is just a 50dB pad straight into the mic transformer which, in combination with the first stage has a gain of 50dB for an overall 0dB gain. Problem is, with a nominal +4dBu input and hot tape levels peaking 15dB above this, the first stage output could easily peak at nearly +20dBu during mixdown which it probably not designed to do.

When tracking and mixing sources direct to tape, the operating levels are likely to be much lower, especially as many of these boards were used with ribbon mics.

Cheers

Ian

abbey road d enfer · Jan 19, 2011

I think the most interesting aspect of these modules is that they were designed around the concept of high open-loop gain and serious NFB.
Apparently NFB was not as demonized as it is today.
No DC heaters...
And the tubes are of such pedestrian pedigree...

It can't work!

lassoharp · Jan 19, 2011

I've been thinking about whether they could have used a different EQ section with a more straightforward amp. Something like the Langevin or Altec constant Z type or even a version of the Pultec type. The need for the extra makeup gain here may have thrown a wrench in the bussing design, plus maybe an extra transformer would have been needed.

It's hard to tell on the cost of one vs the other. The UA EQ as is already requires a separate housing and special tapped inductor. Doesn't seem too far from the other types in terms of space/cost.

Any opinions on whether it's considered good design to go for 9db of hi/low boost(from flat) by means of a FB loop?

emrr · Jan 19, 2011

abbey road d enfer said:
I think the most interesting aspect of these modules is that they were designed around the concept of high open-loop gain and serious NFB.
Apparently NFB was not as demonized as it is today.

Almost every modular recording/broadcast in the USA from 1950 on used very large amounts of open loop gain and large NFB. In that regard they strike me as being like their kin. Most here were pentodes rather than triodes, with triode preamps having heavy NFB sneaking in around 1955 with the RCA BA-21A and the Altec 428. Langevin stuck with the pentode approach until 1961. I don't see a year of reference here, but would note the cascode input stage is otherwise unknown in USA amps, likewise the use of variable EQ in the FB loop.

ruffrecords · Jan 19, 2011

emrr said:
C7 in the 101D is a touch I don't remember seeing before either.

I don't see a C7 in the 101 but there is one in the 100. Is that what you meant?

Cheers

Ian

emrr · Jan 19, 2011

It's there. Above and to the left of the input tube, goes from V2 cathodes to T1 CT.

ruffrecords · Jan 19, 2011

abbey road d enfer said:
I think the most interesting aspect of these modules is that they were designed around the concept of high open-loop gain and serious NFB.
Apparently NFB was not as demonized as it is today.

Indeed, I think the hatred of NFB is a fairly modern audiophool thing. In the early post war years most designs employed NFB. Have a look at the RCA broadcast consoles designs and they are full of it.

That said, NFB in tubes is non-trivial especially at very low frequencies and it is easy to get wrong. With semiconductors it is easy to close the NFB loop at dc and eliminate these problems. This is much more difficult with tubes and I can think of only one tube mic pre design that manages it.

Cheers

Ian

ruffrecords · Jan 19, 2011

emrr said:
It's there. Above and to the left of the input tube, goes from V2 cathodes to T1 CT.

Ah, yes, now I see it!

Thanks,

Ian

ruffrecords · Jan 19, 2011

lassoharp said:
Any opinions on whether it's considered good design to go for 9db of hi/low boost(from flat) by means of a FB loop?

Go to the group gmail account and download the complete info. There's a discussion in there about the reasoning behind the design choices.

Cheers

Ian

emrr · Jan 19, 2011

ruffrecords said:
NFB in tubes is non-trivial especially at very low frequencies and it is easy to get wrong. With semiconductors it is easy to close the NFB loop at dc and eliminate these problems. This is much more difficult with tubes and I can think of only one tube mic pre design that manages it.

Do tell; which do you refer to?

ruffrecords · Jan 19, 2011

emrr said:
ruffrecords said:

NFB in tubes is non-trivial especially at very low frequencies and it is easy to get wrong. With semiconductors it is easy to close the NFB loop at dc and eliminate these problems. This is much more difficult with tubes and I can think of only one tube mic pre design that manages it.

Click to expand...

Do tell; which do you refer to?

Actually I was going to say the Pultec MB-1 but looking again at the schematic, even that does not close the loop at dc. It does, however, have only a single pole in the open loop so it is unconditionally stable.

So, I have to retract my earlier statement and say I no of not one tube mic pre that closes the NFB loop at dc.

Cheers

Ian

Original UA console - 100D preamp, EQ, 101D program amp

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