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General Discussions => The Lab => Topic started by: blackbox on August 05, 2004, 02:24:52 AM

Title: low input gain on UA 176
Post by: blackbox on August 05, 2004, 02:24:52 AM
anybody familiar with the UA 176?
Great sounding box. This one has a very low input gain requiring me to really have to crank the knob or crank the mic preamp resulting in distortion. Any ideas out there? Could this be an impedance or balanced vs unbalanced issue?
think ua will ever reissue these? they're the best compressor ever imho.
Title: low input gain on UA 176
Post by: toby on August 05, 2004, 05:04:31 AM
never used one but from the schematic it should have loads of gain. that 2 stage class b output amp should have loads of gain.

what pre were you using to drive it? if it is feeding it unbalanced (10k) it could be that if the input of the UA is 600 (high to lo don't go....)

finally how old are the tubes? seems obvias but the can loose loads of gain if there abit past it.  

i know when people have had the 600 ohm input problem with the LA2 the have added a 2nd 10k input transformer.

toby
Title: low input gain on UA 176
Post by: blackbox on August 05, 2004, 08:07:18 PM
My patchbay is unbalanced, I believe.
It's a Neotek Elan Console.
Maybe this has something to do with it?
Title: low input gain on UA 176
Post by: Bjorn Zetterlund on August 05, 2004, 08:30:18 PM
Balanced/unbalanced is only ever 6dB difference, is your problem worse than that?  I've always been curious why these old beasts are never anyone's varimu pet...just seems like no one ever talks about them, so I'll stay tuned to this one!

Bjorn

An output that isn't balanced will still have a low output impedance if it is anything 'recent', as in an op amp will usually have a resistor of 50-100R after it.  That doesn't mean it can or cannot drive a 600R input; a TLO71 couldn't whereas a NE5534 could.  The LA2A mod works because what the input transformer sees is a fairly low impedance load.  Rather than using the original 1:10 TRF (basically a gearbox at the input which makes the input harder to drive, but allows us to drive it to higher levels), we can use a 1:1 TRF,  which makes it much easier to drive (even for modern 'wimpy' output stages), but means we have to give it more gain at the output stage; but apparantly this is not a problem.  Just to clarify, there is no 'second' transformer added before the original, rather it is swapped.
Title: low input gain on UA 176
Post by: SSLtech on August 05, 2004, 10:59:09 PM
Elans are unbalanced everywhere apart from the mic inputs.

Make sure that you're using pins 2 and 3 on the preamp and ignoring pin 1. Use pins 1 and 2 only, and you have a recipe for low gain with distortion. The fact that you have distortion (presumed to be clipping) indicates that you don;t have "low gain".... you have plenty of gain, but a low signal. That suggests an imperfect hookup, and those bloomin' Neotek elans are famous for it.

Something else to eatch out for. The Elans don't have DC blocking caps on their outputs. They will leak a little DC into any transformer couled gear that you hook them into. Over time, they do damage.

The Elan sounds pretty good. -Unfortunately it is riddled with silly shortcuts. Add some caps to the insert sends and tape outs, etc. -or pay the price.

Keith

Keith
Title: low input gain on UA 176
Post by: blackbox on August 06, 2004, 02:11:21 AM
ssl tech,
thanks but i'm not following you very well. ignorance on my part im sure.....
so.... the 176 is fine? I'm coming out of the preamp (1073, api, whatever) via the patchbay and going into the 176. I have to crank the Input a lot. Seems like more than the 6db of difference that Bjorn mentioned. If it is just a matter of 6 db, is this really acceptable. If the Elan is unbalanced everywhere, then is all of the rack gear dealing with a gain structure nightmare? I only use the board for monitoring while tracking and avoid the inserts while mixing by patching gear in after the Tape Machine Outputs and then into the Line Inputs. Does this theoretically avoid the problem?
Title: low input gain on UA 176
Post by: toby on August 13, 2004, 04:32:29 AM
Quote from: "Bjorn Zetterlund"
 Just to clarify, there is no 'second' transformer added before the original, rather it is swapped.


i don't see why, addind a second input transformer makes the unit way more versatile, but i suppose nobodys compaining about having a spare UTC laying around! :grin:


with a normal level coming from the desk what are the meter readings for GR, input and output?


Toby
Title: low input gain on UA 176
Post by: jhaible on August 13, 2004, 09:40:46 AM
Quote from: "toby"
i know when people have had the 600 ohm input problem with the LA2 the have added a 2nd 10k input transformer.
toby


I have the same problem with my LA2-based compressor. I should have listened to you people and built in a 1:1 (10k) transformer right away!

Instead I have this O.E.P. with 600R : 25k .
As long as connect a synth with low output impedance (Rhodes sound from my WSA-1), everything works fine. But when I connect my (real) Wurlitzer E-Piano, the LA2 input loads the Wurlie too much (dividing down its output voltage), and even though the transformer amplifies that voltage by factor 6.45, the overall noise level is much increased as well.
When I run the Wurlie thru a cheap guitar stomp box (Ken Multi Phaser (;->)), it's ok again - this can drive the 600R input. Still not optimal at low frequencies, I guess.

So sooner or later, I will put a 10k:10k in there. Maybe I'll use an extra tube gain stage before the optoelectronic voltage divider, to get a similar
gain as the original, but at a higher input impedance.

Generally, the more I think about building "pro" gear, the more I come to the conclusion that on the outside, the small power stage driving a 600R load is quite useful (behaviour of drivers and transformer at higher levels of power etc.), but on the input side, the 600R are quite annoying (unless your whole signal chain is designed for 600R, of course).

No wonder that you find a lot of recommendations for higher impedance / lower gain input transformers for cloned circuits.

Which leaves the question how to deal with the acompanying loss of input gain. Often this may not be a problem - but then again, the original designers certainly had their reason to implement some extra gain reserve. If I want to use one of these classic limiters just for that: limiting signal peaks, it won't be much of a problem (at least I hope so). But compressors can also be a wonderful creative tool when working at their GR limits (great for solo instruments complete with echo and reverb, all run thru maximum GR), so I should really consider an extra active gain stage when I huse a 1:1 input transformer.

JH.
Title: low input gain on UA 176
Post by: CJ on August 13, 2004, 02:56:12 PM
Funny, I just got the 175 schematic yesterday.
There might be some jumpers inside the box to allow for gain tweaking, as Putnam was trying to make that box very versitile.
There sure is a weird output transformer in some of the revs, a bunch of taps are used to control compression, instaed odf a pot.
cj
Title: low input gain on UA 176
Post by: toby on August 13, 2004, 03:45:10 PM
i think its the 176 and 177 that have that crazy output transformer. the 175 one is pretty standard.
Title: low input gain on UA 176
Post by: jhaible on August 13, 2004, 06:35:36 PM
Quote from: "toby"
i think its the 176 and 177 that have that crazy output transformer. the 175 one is pretty standard.


What would the 175 output transformer be like?
10k CT : 600 ?

JH.
Title: low input gain on UA 176
Post by: CJ on August 13, 2004, 07:16:28 PM
I don't know. PRR has me thinking.
especially on the 670 compressor signal output. I was guessing Triad HS-50, a plate to line, but if the 6386's are running down ther towards the bottom most of the time, and since there are 4 in parallel , maybe they can get away with a HS 50 line to line. Just thinking out loud, sorry. And I know you are talking about the output, not the middle trans.

Yes, probably 10k to 600.

cj
Title: low input gain on UA 176
Post by: soundguy on August 13, 2004, 08:00:44 PM
JH-

you are patching the EP directly to the La2a?  Its true that an La2 has lots of gain, but you really should be plugging your EP into a DI first and THEN a mic pre before the la2a...  

dave
Title: low input gain on UA 176
Post by: jhaible on August 13, 2004, 08:18:15 PM
Quote from: "soundguy"
JH-

you are patching the EP directly to the La2a?  Its true that an La2 has lots of gain, but you really should be plugging your EP into a DI first and THEN a mic pre before the la2a...  

dave



A mic preamp would be overkill - the output is quite hot - it's a Wurlitzer - these have a preamp already built in. Just too much output impedance. A simple emitter follower / cathode follower / opamp buffer of a cheap guitar pedal is enough.

But I have heard of people who throw out the electronics of their Wurlizters completely, and then run the (capacitive) pickup directly into a Mic preamp. In that case, the Mic amp's phantom power will replace the (normally muchhigher) internal DC bias voltage across the Wurlie's pickup. But that's surely a special cas. (And I haven't tried it myself.)

JH.
Title: low input gain on UA 176
Post by: soundguy on August 13, 2004, 08:23:52 PM
I always record a wurli with the instrument out to a DI, just like a rhodes.  Yeah, theres an amp, but on both of mine, the headroom is kind of sucky and it always sounds better running it at a lower level and sticking with a more "conventional" gain structure.  Might just be the aging state of my wurlitzers though, but figured I'd comment anyway.

dave
Title: low input gain on UA 176
Post by: jhaible on August 13, 2004, 08:34:03 PM
Quote from: "soundguy"
I always record a wurli with the instrument out to a DI, just like a rhodes.  Yeah, theres an amp, but on both of mine, the headroom is kind of sucky and it always sounds better running it at a lower level and sticking with a more "conventional" gain structure.  Might just be the aging state of my wurlitzers though, but figured I'd comment anyway.

dave


I also use the preamp output, and yes, not much headroom there. But how do you avoid that internal preamp's clipping by something you add afterwards in the signal path?

JH.
Title: low input gain on UA 176
Post by: soundguy on August 13, 2004, 08:56:31 PM
well, you avoid clipping the preamp in the first place...  I usually keep the level at about as low as I can make it go and get something usable which is generally at about 9 o'clock or so.  Bringing up the gain on mine just add a bunch of garbage that isnt too complimentary, but it all goes kind of unnoticed if you plug into a guitar amp directly in which case I'll drive the hell out of the amp...  But for a clean chimey wurlitzer sound, I always keep the volume as low as I can, you can really beat on the keyboard too and not have to worry.  I guess that sorta reeks of modifying the output driver on the amp which I guess is where the problem is since the input is typically clean no matter how hard you hit the keys.  Bigger fish to fry for me though.  If you cant get a good sound out of a wurli, before you go and modify other gear to accomodate it, I would suggest just trying a DI first, that might save you a TON of trouble.  Plus with a DI, there's really not too much special abou the solid state amps in the wurlitzers, its much cooler to use the pre of your choice for the color.  Id vote for a passive wurlitzer just like a rhodes if you are just using it for recording.

dave
Title: low input gain on UA 176
Post by: blackbox on August 14, 2004, 02:46:42 AM
OK whatever. Back to the 176......
I've heard about the jumpers before. Anyone know more about them and where the are located? Someone told me a couple of years ago I think that there are jumpers that change the operating levels. I forgot all about it until cjenrick mentioned it.

BTW, wurlys sound best recorded acoustically with a 57 on each speaker.
Nast and grungy with the sweetest stereo tremelo ever.
Title: low input gain on UA 176
Post by: jhaible on August 14, 2004, 04:13:50 AM
Quote from: "soundguy"
well, you avoid clipping the preamp in the first place...  I usually keep the level at about as low as I can make it go and get something usable which is generally at about 9 o'clock or so.  


I thought the volume potentiometer was connected after the preamp inside the Wurlie, but it has been a long time since I have looked into the schematics. I'll check that again.

JH.
Title: low input gain on UA 176
Post by: soundguy on August 14, 2004, 02:07:42 PM
Quote from: "blackbox"
BTW, wurlys sound best recorded acoustically with a 57 on each speaker.Nast and grungy with the sweetest stereo tremelo ever.


Gee, now that I have this little tidbit I guess I can stop experimenting now that someone has figured out how something sounds "best"...  Just as an illustration of different tastes, I dont think there is a sound on earth that sounds "best" with a 57.  Plus, if you are using a dynamic on the speakers, if you compress the keys alot you tend to bring up a bunch of keyboard finger noise with it and it just sounds clunky...  Everything is so dependant on the recording at hand, there isnt a best way to do ANYTHING...

JH- Im not sure how the circuit is wired, but those results always worked for me.  It would be nice to figure a passive way out of that thing to just use a real amp on it in the first place.

dave
Title: low input gain on UA 176
Post by: jhaible on August 14, 2004, 04:05:39 PM
Quote from: "soundguyJH- Im not sure how the circuit is wired, but those results always worked for me.  It would be nice to figure a passive way out of that thing to just use a real amp on it in the first place.

dave[/quote


Haven't looked it up yet. I should know, as I have re-built the whole preamp when I have bought it (PCB was mechanically broken). My Wurlie is a 200A, BTW.

What I said about "passive" is just what someone told me. Never tried it.
IMO, the best way to amplify a Wurlitzer would be an internal tube amplifier. And the first Wurlitzers were just built that way, but I don't have these schemos.

Apart from that, IMO the best way to amplify a Wurlitzer would be to get rid of the electrostatic pickup at all. (;->) The idea to just apply a high polarising voltage to the mechanical construction, and using that as a capacitive pickup, is certainly brilliant at fist glance, but in practice its tendency to attrackt dust, which in turn causes terrible noise, makes it a rather bad solution.

But on a day with relatively low noise, it makes the most wonderful sounds, and it has this excellent keyboard action.

If I could get rid of that permanent noise problem (SNR, if measured in peak voltage, is actually *negative* on my Wurlitzer - the noise peaks are louder than the signal on a bad day ...), I would build a tube preamp for that instrument immediately.

JH.
Title: low input gain on UA 176
Post by: soundguy on August 14, 2004, 09:35:22 PM
brad has a schematic on his site which is worth a look-

http://www.avensonaudio.com/tech/Wurlitzer/schematic%20rev-A.jpg


dave
Title: low input gain on UA 176
Post by: jhaible on August 15, 2004, 05:37:01 AM
Quote from: "soundguy"
brad has a schematic on his site which is worth a look-

http://www.avensonaudio.com/tech/Wurlitzer/schematic%20rev-A.jpg


dave



Interesting.
Is this the original circuit?
Looks more like a modern clone ...
I would be very surprised if they had LM317-based DC heating in these old Wurlies.

JH.
Title: low input gain on UA 176
Post by: jhaible on August 16, 2004, 07:30:37 AM
On the Wurlitzer sub-topic:

Looked thru my schematics at home.

The 200A has its level pot after the preamp - so reducing the volume will not change the signal level in the preamp at all.

Tremolo is performed with a LDR in the feedback path of the preamp. Tremolo *will* change the signal level in the preamp. (On my 200A, the preamp clips with high tremolo settings. I guess I have to reduce the gain, or the pickup DC voltage. Of really build a tube preamp.)

The 140 (tube model) has a very different architecture. Volume is controlled by a shunt potentiometer *before* the grid of the first tube.
Also, the low level output is derived from that point with a 1Meg resistor. So apart from the internally generated pickup bias voltage, using this output means running the Wurlie passive. And very high impedance.

In that tube model, Tremolo is done with a vary mu methiod in the power amp. (No miking the speakers, no tremolo.)

Tubes are 9k11 triple triode for preamp, phase splitter and tremolo LFO. Plus 2 power tubes in pp.

I guess there are other models that even have other circuit architectures.
(For instance, I know there were at least 4 different ways to make the tremolo VCA: Vari Mu, Diode, Vactrol, and Varistor pair.)

Maybe I should really build a tube amp into my Wurlie.

Maybe get rid of that transistor preamp (it doesn't have a power amp anymore) and just build a single tube stage + transformer as a kind of internal DI box. Or clone that 140 circuit, complete with vari mu tremolo.

No idea if keeping the original 9k11 is a good idea or not. I've started another thread about that.

JH.
Title: low input gain on UA 176
Post by: SSLtech on August 16, 2004, 07:47:35 AM
Quote from: "blackbox"
ssl tech,
thanks but i'm not following you very well. ignorance on my part im sure.....
so.... the 176 is fine? I'm coming out of the preamp (1073, api, whatever) via the patchbay and going into the 176. I have to crank the Input a lot. Seems like more than the 6db of difference that Bjorn mentioned. If it is just a matter of 6 db, is this really acceptable. If the Elan is unbalanced everywhere, then is all of the rack gear dealing with a gain structure nightmare? I only use the board for monitoring while tracking and avoid the inserts while mixing by patching gear in after the Tape Machine Outputs and then into the Line Inputs. Does this theoretically avoid the problem?

It depends. It depends how you wired it and it depends on what the output of each device is. Servo driven outputs behave differently to 'dumb' electronically balanced outputs, which behave differently to transformer outputs.

"Losing 6dB" is a vast over-simplification ... I direct the Jury to disregard that suggestion and ask that it be stricken from the records.

The fact is that if you have a transforemr output, you *MUST[/u][/b]* make sure that both pins 2 and 3 are connected to the unbalanced device. Otherwise you'll lose a LOT more than 6dB, and the sound will be thinner than Tony Iommi's top string! So many people think that you connect an unbalanced signal to hot and ground on a balanced piece of gear, that this crops up a lot. -Connect to hot and cold. Ignore ground more often than not, and only go back and connect it if you have a buzz. (*NOTE* the difference between a buzz and a hum... a hum is most often too many grounds, a buzz is usually too few grounds.)

If that don't get it, it's something else... (Hmmmm.. that sounded so much 'wiser' in hy head!  :wink: !!!)

Keith
Title: low input gain on UA 176
Post by: gyraf on August 16, 2004, 07:58:30 AM
Quote from: "SSLtech"
NOTE* the difference between a buzz and a hum... a hum is most often too many grounds, a buzz is usually too few grounds.)


That's a good one!
Title: low input gain on UA 176
Post by: on August 16, 2004, 08:31:31 AM
On the Wurlitzer Subtopic, Can you post a schematic of the 140 with vari-mu tremolo?
Title: low input gain on UA 176
Post by: jhaible on August 16, 2004, 09:05:39 AM
Quote from: "thomasholley"
On the Wurlitzer Subtopic, Can you post a schematic of the 140 with vari-mu tremolo?


I only have it in paper form, but I can scan it and put it on the web.
Not tonight, though. (I'm looking forward to a nice evening with MAM's Stefan Schmidt and Lintronics' Rudi Linhard in my little studio tonight. 8-) ).

The tremolo is just adding a common mode LFO signal to the differential mode audio signal on th epower tubes' grids. I guess this counts as variable Mu, but it's quite common in guitar amps AFAIK.

JH.
Title: low input gain on UA 176
Post by: CJ on August 16, 2004, 06:05:18 PM
blackbox, PM jh for some 175/6 info.
cj
Title: low input gain on UA 176
Post by: jhaible on August 17, 2004, 06:33:55 PM
Quote from: "thomasholley"
On the Wurlitzer Subtopic, Can you post a schematic of the 140 with vari-mu tremolo?


http://www.oldcrows.net/~jhaible/wurlie/jh_wurlitzer_145.pdf

The model number is 145, not 140 - my mistake.

The scan isn't beautiful, but it should be readable.

JH.
Title: low input gain on UA 176
Post by: on August 17, 2004, 10:54:57 PM
Thanks a lot for the schematic.