TAB V72a as active summing amps?

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RFSiesta

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Feb 20, 2015
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Graz, Austria
We use either V672 and V672/2 (slightly less coloured) as summing amps in our Trident 80 for years now, with huge success - and remember someone mentioning the V72a to be a great summing amp, too: I understood he meant active (current) summing, because they also have a trident console.
(we have two racked ones as mic pres, but they are not the same league as a V72 or V76, so they don't get too much use) - I studied the manual and they say it's a "voltage amplifier", with a 4kΩ input impedance :/
Is configuration as low-Ω amplifier possible, by wiring or change in circuit? The 672 are completely different, they have an inverting stage with feedback and 2,5-4Ω input impedance. So wiring as summing amps is good, for use as mic preamps they need surrounding circuit with changeable serial resistance, that inevitably leads to decreasing input impedance at high gain, so use as preamps is compromised, while summing currents is not.
Someone told me to use the V72a in this exact application, but I can not imagine how.
Did anyone actually do it?
Schematics are at http://audio.kubarth.com/rundfunk/index.cgi, search "V72a". Can't directly link those docs here.
cheers
 
I believe that the 672  with it's very low input impedance is a completely different concept than the V72a - though it shares the '72' in it's name. Typically all '72'-type modules would have the same basic function, but these versions of the 672 seem to be the exception. I don't see how a stock V72a would work as a current summing amp unless the input transformer was changed. Then again, with an appropriate resistor in parallel to the input it would work as a makeup amplifier in a passive mix bus - maybe that's what the idea  was?

Michael
 
I am confused. The V672 is a transistor based transformer input zero ohm amplifier ideal as a virtual earth summing amplifier.

The V72a is a tube based mic pre. Not similar in any way.

Cheers

Ian
 
[silent:arts] said:
It is a Studioverstärker, It was used as a make up gain amplifier after passive filters, not as a mic pre  ;)

According to my copy of  Tonstudiotechnik it was used as a mic pre for condenser mics. It was certainly used as one n the early EMI desks.

Cheers

Ian
 
Here some quotes from the V72(a) manuals:

"It is a voltage amplifier.."

"it serves primarily as a pre, intermediate and main amplifier"

"to compensate the basic attenuation of a filter" (in microphone channels with V76 in front, "without sacrificing the V76's overdrive resistance" )

"due to the inherent noise caused by the high input resistance of the V72a the use as a microphone amplifier is only recommendable with condenser microphones"

V72 is a universal amp, you can use it for nearly everything, even as microphone preamp, if the gain is enough for your application.
 
Thank you, that solves the whole issue.
We use them as mic pres, they are cool for rock vocals and some other stuff, but tend to cut off the lows and HF, too - might be worth the time tweaking them for this purpose, there seem to be ways to improve bandwidth.

If it was only from the manuals I found, there is no connection between 672 and 72 except the "72", but I got the information two times that people claimed to use it as summing amp - and we racked a couple of 672 as mike preamps, too.

Maybe someone went from his bus outs through the 72a to increase gain or induce colour - or use it as a passive summing amp with less than 10 channels or so...
There's obviously plenty of nonsense info around, thank you all for clarification!

Thread closed, afaiac ;)
 
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