CurtZHP

Re: Interstage curious
« Reply #60 on: December 02, 2019, 08:03:47 PM »
So, digging through the parts stash, I found a pair of brand new 56K 2W resistors.  I couldn't *resist*.   ::)

Might 85dB of gain suggest that I was "overcompensating" for something? 
Electrons don't read schematics.


EmRR

Re: Interstage curious
« Reply #61 on: December 02, 2019, 08:19:41 PM »
Yeehaw.  Now we're talkin' 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Re: Interstage curious
« Reply #62 on: December 02, 2019, 10:31:37 PM »

It looks like I have the basic circuit figured out.  Just need to finalize some component values on those first two stages.

Have you tested that circuit with a square wave generator to look at "ringing" or other artifacts?  Three transformers is a lot of iron... UTC transformers are good, but if the circuit will pass clean square wave over the audio spectrum, the integrity of the preamp to transients should be good.  The sine wave tests say something, but you need to go a little further.  As an experiment, take the input transformer and feed -10 dbm of square wave in, terminate the secondary with a 100 k resistor, put a scope across the output, and look at the wave form.  If you have a dual trace scope and can look at the input and output simultaneously, what you may see may surprise you.  You may observe improvement by changing the 100k resistor to a lower or higher value.  "Tuning" out any "ring" with a low value capacitor may also help.  Remember, you have Miller effect at the input, especially a input stage with gain.  Miller effectively multiplies the effective capacitance at the grid by the tube section gain, so be careful. Keep the leads short and stray capacitance at a minimum...

PRR

Re: Interstage curious
« Reply #63 on: December 02, 2019, 10:47:06 PM »
> only get about 50dB of clean gain out of this before it gets ugly.  ...That's with 100mV signal going in

0.1V, up 50dB, is 30 Volts out. Where do you need a 30V signal?? It's not even legal on a telephone line.

I would not use 50dB gain for anything bigger than 3mV. 50dB is too much for dynamic mike for a PA speech mix. Unless you only do ribbons on harpsichord, you almost never want an uncontrolled 50dB gain. Classic tube mixer preamps were 35dB or 40dB. With hot modern acts and mikes, even that is over-much.

Sig-gens mostly don't turn-down that low. You probably want a 40dB pad between signal generator and mike input. 5k, 100r, 5k.

CurtZHP

Re: Interstage curious
« Reply #64 on: December 03, 2019, 06:59:28 AM »

0.1V, up 50dB, is 30 Volts out. Where do you need a 30V signal?? It's not even legal on a telephone line.


Trust you to take all the fun out of it.   ;)
Electrons don't read schematics.

CurtZHP

Re: Interstage curious
« Reply #65 on: December 03, 2019, 07:00:07 AM »
Have you tested that circuit with a square wave generator to look at "ringing" or other artifacts?  Three transformers is a lot of iron... UTC transformers are good, but if the circuit will pass clean square wave over the audio spectrum, the integrity of the preamp to transients should be good.  The sine wave tests say something, but you need to go a little further.  As an experiment, take the input transformer and feed -10 dbm of square wave in, terminate the secondary with a 100 k resistor, put a scope across the output, and look at the wave form.  If you have a dual trace scope and can look at the input and output simultaneously, what you may see may surprise you.  You may observe improvement by changing the 100k resistor to a lower or higher value.  "Tuning" out any "ring" with a low value capacitor may also help.  Remember, you have Miller effect at the input, especially a input stage with gain.  Miller effectively multiplies the effective capacitance at the grid by the tube section gain, so be careful. Keep the leads short and stray capacitance at a minimum...

Will do.  Thanks!
Electrons don't read schematics.

CurtZHP

Re: Interstage curious
« Reply #66 on: December 03, 2019, 08:35:10 PM »
So I ran a square wave, and yes, there is some apparent ringing.  I tried the tuning trick with a few different capacitors I had on hand.  While they tuned out the ring, the trade off was predictably some high end roll-off.  Anything under 300pF seemed to do the least damage in that regard, but then only made a small dent in the ringing.  I suppose there's a trade-off somewhere.

This was all just on the input transformer.  I haven't had a chance to check the others.

Read some interesting stuff from a guy from Jensen about this issue.  I'll have to dive deeper into this.

How much ringing is too much?  How much is "close enough for rock and roll?"
Electrons don't read schematics.

EmRR

Re: Interstage curious
« Reply #67 on: December 03, 2019, 09:06:22 PM »
Any and all is close enough for rock and roll.   Really.  Get the amp right first.  That's the LAST thing you ever have to worry about.  You think any vintage preamps have ringing tuned out?  Are they getting any less desirable on the used market?   Sure, look at it at some point, AND listen. 
« Last Edit: December 04, 2019, 02:37:17 PM by EmRR »
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

CurtZHP

Re: Interstage curious
« Reply #68 on: December 04, 2019, 07:05:42 AM »
Any and all is close enough for rock and roll.   Really.  Get the amp right first.  That's the LAST thing you ever haver to worry about.  You think any vintage preamps have ringing tuned out?  Are they getting any less desirable on the used market?   Sure, look at it at some point, AND listen.

Point taken.  I wasn't going to lose too much sleep over it anyway.  Like you suggest, this thing is old school iron and tubes, and it's kind of supposed to have a "sound" to it.

The ring test was more an academic exercise for me.  I had never thought about it before, and it was interesting to see what would happen.

I think I'm about 90% of the way there as far as the rest of it goes.  Just need to swap out a component or two to get to the correct values.  I'll have to order some parts.  And I need to get rid of a few clip leads and replace them with shorter, more permanent connections.  Generally clean things up a little more.
Electrons don't read schematics.

CurtZHP

Re: Interstage curious
« Reply #69 on: December 07, 2019, 04:46:40 PM »
So, this weekend I finally got the guts to actually connect a mic to one end and a headphone amp to the other.  Plugged in a dynamic mic and all was well.  Then I bodged together the parts between the phantom supply and the mic input to test it with a condenser.

Passes signal just fine, but now there's this nasty little hum underneath it.

I need to dig out my copy of "Grounding 101" by our very own ruffrecords, and study it carefully.  I obviously have something amiss here.
Electrons don't read schematics.


CurtZHP

Re: Interstage curious
« Reply #70 on: December 09, 2019, 08:30:22 AM »
After looking over my notes, and revamping some messy ground wires, I got think I got rid of the hum as long as I don't hook up phantom power.  As soon as I engage that and plug in a condenser, hummmmmmmmm........

It's slight, but present.

I've attached a schematic that better outlines (I hope...) what I did as far as grounding.  The only thing I didn't include might actually be the culprit.  But, not having the project in front of me at the moment, I can't say with certainty how it's connected.  I'm referring to the AC mains ground wire.

I'm almost certain it's currently (no pun intended...) connected to the 0V run on the power supply.  (The power supply here is an exact repeat of the supply I built for my last tube project, so it's already on a PCB.)

Since this project is still just a pile of guts on the workbench, there is no "chassis" to speak of.  I'm presently using the shield ground pins on the transformers to stand in as the "chassis."  Is it a mistake to do this? 

I have a sneaking suspicion my mistake is not connecting the mains ground to the "chassis" instead of the 0V of the power supply.

Am I correct as far as what I've designated as 0V (signal ground) on the audio circuit?
Electrons don't read schematics.

EmRR

Re: Interstage curious
« Reply #71 on: December 09, 2019, 11:19:41 AM »
Surely a result of having no chassis. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

CurtZHP

Re: Interstage curious
« Reply #72 on: December 09, 2019, 11:27:01 AM »
Surely a result of having no chassis.


Yeah, nobody likes it when I run around naked either.   :-[
Electrons don't read schematics.

EmRR

Re: Interstage curious
« Reply #73 on: December 09, 2019, 11:56:47 AM »

Yeah, nobody likes it when I run around naked either.   :-[

I get that all the time around the house here.     Maybe I should take it outside and see if it goes better?
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

dmp

Re: Interstage curious
« Reply #74 on: December 09, 2019, 12:28:23 PM »
The power ground should go to the chassis (the transformer ground pins in your situation) and the other grounds should all connect together there as well (0v, audio, filament, phantom).
 

CurtZHP

Re: Interstage curious
« Reply #75 on: December 09, 2019, 01:08:46 PM »
I get that all the time around the house here.     Maybe I should take it outside and see if it goes better?

(Trying to find the "unsee" emoticon....)   ;D
Electrons don't read schematics.

CurtZHP

Re: Interstage curious
« Reply #76 on: December 13, 2019, 08:39:20 PM »
So here's a question....

Should I have some sort of B+ decoupling on the feed to the output transformer primary center tap?

While trying to chase down the hum (which I've somehow managed to make worse!), I started nosing around with the test probes and found about 23VAC between that center tap and each side of the primary.  Never saw it before, because I hadn't looked for it before.  That's with the gain control all the way down, so there shouldn't be any signal there.

I do have a large cap decoupling the B+ to the first tube stages.
Electrons don't read schematics.

CurtZHP

Re: Interstage curious
« Reply #77 on: December 16, 2019, 12:14:27 PM »
Over the weekend, out of frustration, coupled with a lack of better things to do, I completely dismantled the whole thing, except for the power supply.  (The supply is a separate build, essentially a copy of a supply from my last successful project...)

Redoing the whole thing on a piece of perf board to try to keep things neat.  Going to redo the whole grounding scheme from the "ground" up too.

 :P
Electrons don't read schematics.

EmRR

Re: Interstage curious
« Reply #78 on: December 16, 2019, 02:08:39 PM »
So here's a question....

Should I have some sort of B+ decoupling on the feed to the output transformer primary center tap?


Many times there's none, as it's really 'same as' the output smoothing cap in the PSU
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

CurtZHP

Re: Interstage curious
« Reply #79 on: December 16, 2019, 05:08:37 PM »

Many times there's none, as it's really 'same as' the output smoothing cap in the PSU


That's part of the reason why I was asking.  It seemed redundant.
It seemed to help on my last tube project, but that was only on the first stage, and that was a 12AX7. 
Electrons don't read schematics.


 

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