Q on PP & current mirrors

JoshK · 14th March 2007, 10:12 PM

I was reading through some back issues on tubecad and came across JB's articles on using SS in tube circuits to do "house cleaning" duties. The idea that really struck me was using current mirrors in the cathodes of PP pairs.

In fact, this seems like such a great idea (to my newby thinking) that I wonder why it has not been used/talked about/adopted more widely. Can anyone see why this might *not* be a good idea? It seems to avoid the issue with using a CCS in the tails of a PP pair in that this would be limited to class A only, whereas the current mirror is not.

For the driver/input circuit, CCS is fine and dandy and I am not questioning that. Class A for these stages thank you very much, but for the output stage, it seems like the current mirror would be the superior choice. It also seems like it would keep balance in the pair in case of non-matching, drifting, or other variance.

In my case, I am planning a PP 6c33c amp and I've read of the troubles in matching low rp/high gm tubes such as these. CCS in the tails is one way to force dynamic matching of current through the pair, but I am concerned with what happens when the amp is driven beyond strict class A and am wondering if a current mirror is a better choice (and setting bias current seperately of course).

I will still likely plan for class A, but I'd like to have some safety in how the amp performs when driven hard/to the limit/to clipping. Thoughts will be greatly appreciated.

Josh

Irakli · 14th March 2007, 11:32 PM

Hi,

On paper (or in simulators) a lot of things look perfect. Real life is little bit different.
I think to make high current high quality current mirror is not an easy task.

JoshK · 14th March 2007, 11:44 PM

Quality I don't know about, no experience there. Current though I would have thought would have been easy, since transistors are such higher current devices than tubes.

JoshK · 18th March 2007, 04:06 AM

Anyone else have experience or supposition on the matter?

SY · 18th March 2007, 04:47 AM

What's the looking-in impedance of a current mirror?

andrew_whitham · 18th March 2007, 04:56 AM

Hey JoshK

I too have been looking at this in conjunction with servo 's for setting the bias and allsorts, and I'm still no nearer...

I think that you are right, S-State in support of tubes is a grand idea, you're looking at what 300Ma per side? there are ways of getting a gurrent mirror to deal with that (on paper anyway) cascoding is an obvious one.

BUT here's my question, and perhaps, I'm overthinking this, but I dont think for a P-P stage you need dynamic current matching, rather that the two stages are equal but opposite over a long enough time constant that the music is unaffected? Is that right? (not an expert on P-P output stages, (or any others) complete noob really)

Depending on your bias arrangements then perhaps an easier method of achieving what you need is to build a common reference for the two tubes and use that "somehow" to control the current per tube.

John Broskie's Tubecad site also has a description of a clipping servo for setting bias, which works for either class A or B. THeres also something in Morgan Jones' tome. Too complicated for my little brain to figure though Class A is *way* simpler.

cheers

Andy

JoshK · 18th March 2007, 02:50 PM

Quote:

Originally posted by SY
What's the looking-in impedance of a current mirror?

I take it that was a question to provoke me to answer my own question?

I don't know but I'll have to look into it. But if it were really high and that wasn't good for the cathodes, then you could put the mirror on the plates.

Thanks for the response guys. Food for thought.

SY · 18th March 2007, 04:05 PM

Quote:

Originally posted by JoshK

I take it that was a question to provoke me to answer my own question?

Pass the hemlock, please.

JoshK · 25th March 2007, 09:31 PM

i went back and took another look at the circuit broskie posted. i forgot that the current mirror was bypassed on each leg with caps for the ac signal path.

David Davenport · 26th March 2007, 02:14 AM

Josh,

What issue of tubecad was JB's current mirror article in?

Dave

14th March 2007, 10:12 PM	#1
JoshK diyAudio Member Join Date: May 2004 Location: Toronto, ON	Q on PP & current mirrors I was reading through some back issues on tubecad and came across JB's articles on using SS in tube circuits to do "house cleaning" duties. The idea that really struck me was using current mirrors in the cathodes of PP pairs. In fact, this seems like such a great idea (to my newby thinking) that I wonder why it has not been used/talked about/adopted more widely. Can anyone see why this might not be a good idea? It seems to avoid the issue with using a CCS in the tails of a PP pair in that this would be limited to class A only, whereas the current mirror is not. For the driver/input circuit, CCS is fine and dandy and I am not questioning that. Class A for these stages thank you very much, but for the output stage, it seems like the current mirror would be the superior choice. It also seems like it would keep balance in the pair in case of non-matching, drifting, or other variance. In my case, I am planning a PP 6c33c amp and I've read of the troubles in matching low rp/high gm tubes such as these. CCS in the tails is one way to force dynamic matching of current through the pair, but I am concerned with what happens when the amp is driven beyond strict class A and am wondering if a current mirror is a better choice (and setting bias current seperately of course). I will still likely plan for class A, but I'd like to have some safety in how the amp performs when driven hard/to the limit/to clipping. Thoughts will be greatly appreciated. Josh

18th March 2007, 04:47 AM	#5
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	What's the looking-in impedance of a current mirror? __________________ If there's a sucker born every minute, where do the rest of them come from?

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Complementary input stages with current mirrors	jgedde	Solid State	17	2nd December 2008 03:16 AM
Current mirrors driving FETs	Kevinbd	Solid State	23	1st December 2006 12:50 AM
Current Mirrors and first post here	vynuhl.addict	Solid State	6	16th November 2006 08:43 PM
BCV61/62 monolithic current mirrors	Mr Evil	Parts	0	23rd February 2005 02:45 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

14th March 2007, 11:32 PM	#2
Irakli diyAudio Member Join Date: Aug 2004 Location: washington, DC Blog Entries: 1	Hi, On paper (or in simulators) a lot of things look perfect. Real life is little bit different. I think to make high current high quality current mirror is not an easy task.

14th March 2007, 11:44 PM	#3
JoshK diyAudio Member Join Date: May 2004 Location: Toronto, ON	Quality I don't know about, no experience there. Current though I would have thought would have been easy, since transistors are such higher current devices than tubes.

18th March 2007, 04:06 AM	#4
JoshK diyAudio Member Join Date: May 2004 Location: Toronto, ON	Anyone else have experience or supposition on the matter?

18th March 2007, 04:56 AM	#6
andrew_whitham diyAudio Member Join Date: Oct 2005	Hey JoshK I too have been looking at this in conjunction with servo 's for setting the bias and allsorts, and I'm still no nearer... I think that you are right, S-State in support of tubes is a grand idea, you're looking at what 300Ma per side? there are ways of getting a gurrent mirror to deal with that (on paper anyway) cascoding is an obvious one. BUT here's my question, and perhaps, I'm overthinking this, but I dont think for a P-P stage you need dynamic current matching, rather that the two stages are equal but opposite over a long enough time constant that the music is unaffected? Is that right? (not an expert on P-P output stages, (or any others) complete noob really) Depending on your bias arrangements then perhaps an easier method of achieving what you need is to build a common reference for the two tubes and use that "somehow" to control the current per tube. John Broskie's Tubecad site also has a description of a clipping servo for setting bias, which works for either class A or B. THeres also something in Morgan Jones' tome. Too complicated for my little brain to figure though Class A is way simpler. cheers Andy

25th March 2007, 09:31 PM	#9
JoshK diyAudio Member Join Date: May 2004 Location: Toronto, ON	i went back and took another look at the circuit broskie posted. i forgot that the current mirror was bypassed on each leg with caps for the ac signal path.

26th March 2007, 02:14 AM	#10
David Davenport diyAudio Member Join Date: Jun 2004 Location: Raleigh North Carolina	Josh, What issue of tubecad was JB's current mirror article in? Dave