Universal Audio 175?

[Scenaria]

can anyone in here share any info on this box?

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=23790&item=3734523254&rd=1

 

[wilebee]

they sound great they are a balanced varible mu stage based on a 6bc8 single ended output its alot similar in design as the altec 436

Wilebee

 

[soundguy]

but not too much sonically, they are way more usable than a 436 any day of the week IMO.

dave

 

[wilebee]

agreed much more useable

Wilebee

 

[NewYorkDave]

Interesting to note the similarities between the 175's output section and the UA 1016 preamp (or the Langevin 5116B, for that matter).

At first glance, it seems that the interstage transformer could be replaced with R-C coupling, if desired. The output section has enough open-loop gain (about 500x, maybe more) that it could be adjusted to make up for the voltage stepup that would be lost from eliminating the interstage.

 

[toby]

if you eliminated the interstage transformer it would be basically the altec!

 

[NewYorkDave]

Um, no, not really. Compare the two schematics. The output stages are quite different, interstage transformer or not. Plus, the UA has provision for balancing, which the Altec does not.

 

[PRR]

> it seems that the interstage transformer could be replaced with R-C coupling

With R-C coupling, as it starts to limit, there is common-mode shift that the second stage has to ignore. If you kept the 175's 5K plate resistors, it idles at about 6mA per side but that falls to about 1.5mA to get 6dB GR. 6-1.5=4.5mA, 4.5mA*5K= 22V of Common Mode shift. Hard to get that much without a current source in the next stage. The transformer eliminates the issue.

BTW, there are many R-C coupled limiters and some of them seem to deliberately clip in the second stage grids as they go into GR. If you limit only as emergency protection for a transmitter, transient clipping is easier than a faster attack (higher power rectifier amp). This can also be good "flavor" when trying to boost your average/peak ratio in a dense mix.

 

[dale116dot7]

The RC coupling method requires that the attack time be kept reasonably slow - you'll see most of the RC coupled variable-mu limiters keep the attack time constant slower than about 20 msec or so. They need to be otherwise control feedthrough will make a nice motorboat. The current source method would work but you need that 22V (or more) of compliance in the current source which means a very busy circuit - plus you start to use up headroom since tubes need some voltage drop across them to work. You could do it but the interstage transformer has a very good CMRR and it a much simpler solution and yields better performance. Also, the time constant matching of the RC coupling network gets into there to spoil the CMRR and make a motorboat yet again.

 

[NewYorkDave]

Yep, like I said, "at first glance..."

Take a second glance, as you have done--and I did not--and the reasons for the interstage become apparent :wink:

 

[kent]

Somewhere's around here's I gots a PDF of the whole UA17* series with layouts and such if yer interested. It's always been in the back of my brain to try a stereo version of these.

cheers,
kent

 

[toby]

[quote author="NewYorkDave"]Um, no, not really. Compare the two schematics. The output stages are quite different, interstage transformer or not. Plus, the UA has provision for balancing, which the Altec does not.[/quote]

theres some feedback going on in the UA175's output amp, nothing particularly unusual.