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 New
« 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.

 

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