Analysis of a certain class A driver

Hello,

Below is the schematic of the near class A driver of the largest M$n%ey EQ-box.
It's used as a balanced input and to drive the passive EQ sections (reported to drive 100 Ohms up to +27dBm @ 0.002% THD with +/-15V rails).
It could function well for driving for instance the G-Pultec EQ.

Can anyone confirm this circuit to be correct ?

http://home.hetnet.nl/~chickennerdpig/FILES/Pultec/driver_02_60.jpg



In essence I've copied it correctly 'from source' but was left with a few questions.

For this function other circuits will do fine of course, but I got curious
why they did it different from the usual opamp+NPN+PNP.

So I was wondering if some of these were drawing errors
or some smart moves of the designer.

Here goes:

#1
That 100 Ohm resistor across B-E of the NPN... it looks like
it forms a direct path from opamp to output for lower signals
and only for higher levels the BJT kicks in. Any thoughts ?

#2
The original drawing had the direction of both diodes reversed,
I've flipped them here.

#3
I've boosted the value of the input-coupling caps
- originally drawn as 0.47uF which is too low.

#4
The DC-servo looks a bit 'direct'. I mean, the output is directly
used for the 10k bias resistor. Probably works fine, but why are
the usual servo's not as simple as this one ?

#5
Are those NPN- & PNP-types used indeed MJ3055 & 2955 ?
These are TO-3 devices, looks a bit hefty for just driving 100 Ohms.
(Thinking of using BD139 & BD140)

#6
Hmm, BJT's without any emitter-resistors.

#7
Any other incorrect values or topology-errors ?



Comments on any of the above much appreciated :thumb:

Regards,

Peter

#4:
Padding the output reduces noise/distortion from the servo and moves the cut frequency towards DC.

#6:
No good idea IMO...

#7:
Perhaps R2 10k and short R4?

Samuel

Thanks Samuel,

[quote author="Samuel Groner"]#4:
Padding the output reduces noise/distortion from the servo and moves the cut frequency towards DC.

[/quote]I figure I'd best make a few versions, one as drawn and trying various things in its cousin. Will add padding to the servo then as well, will post a pic later. I didn't realize the padding moves the corner-freq, should re-read some servo-info.

#6:
No good idea IMO...

Click to expand...

Agreed. Maybe they don't mind that the biascurrent can shift, but thats strange.
I haven't even looked inside the box, I don't know if they've thermally coupled the BJTs & diodes.

#7:
Perhaps R2 10k and short R4?

Click to expand...

You want (1) to increase the current draw from the opamp and (2) bias the BJTs richer ? Haven't thought much about that 75 Ohm & 100n yet. (OK, R2 lower, then increase the 100n.)

Thanks,

Peter

It looks like the beta of Q2 is a pivotal parameter in this circuit. Since that is highly variable from device to device and also temperature-sensitive, circuit behavior is a scary proposition :shock:

As my late father used to say, affecting a bit of a Scottish brogue, "I'm aginn it."

Thanks Brad,

[quote author="bcarso"]It looks like the beta of Q2 is a pivotal parameter in this circuit. Since that is highly variable from device to device and also temperature-sensitive, circuit behavior is a scary proposition :shock:

As my late father used to say, affecting a bit of a Scottish brogue, "I'm aginn it."[/quote]
Getting more & more the feeling this circuit has nothing special going for it when compared to the 'usual perhaps more boring approach' but is better avoided as it is now.

W.r.t. Q2, you mean that the idle current is strongly determined by the whims of this PNP-device ? Other you mean another 'bad' mechanism ?

FWIW, a certain manual says:

We use an exceptional op-amp/discrete circuit for the input buffer
in order to drive the 150 ohm (worst case) EQ circuit. Not an
appropriate place for tubes. The first design used a transformer (3:1)
for impedance conversion but it had a 10 dB voltage drop and thus
10 dB more noise. The new input circuit isolates input loading and
allows the tube circuits to be better optimised.

Click to expand...

and:

... not a particularly complicated circuit. The
audio comes in, is converted from balanced to unbalanced and DC
servo'd and given enough power (a few watts) to drive the EQ (and
output in "bypass"). It uses a BB OPA2604 op-amp and a complementary
pair of medium current transistors richly biased into near class A
operation for this.

Click to expand...

Thanks,

Peter



(BTW, please let me know the above quoting is better not done)

I happened to find a file on my harddrive, so sorry, I forgot who once kindly provided the 'original' schematic of this box. Could it be that there are errors in that 'original' ? Or has anyone the opportunity to have a look inside this inact&ve f#lter-box of impr*ssive dim#nsi$ns ?

It is hard to believe the plan is correct as drawn. If so, then it seems to be tuned for negative clipping "sound", since 150K+2N2955 will not give much pull-down in 100Ω.

National has a DSL chip re-rated for headphones that will drive your 100Ω with authority and transparency.

The smaller OverTure loudspeaker amp chips ("Gainclone") will work.

Many headphone drivers will work. The Tori is a power-pig but simple and should be faultless.

Since this is DC-coupled into high-H inductors, you may need lower DC offset than we accept in other work. For studio work, you may be fine selecting a low-offset chip and adding a trimmer. For outdoor gigs, you should probably use a servo with a very low offset chip.

A transformer would be nice here.

[quote author="clintrubber"]Getting more & more the feeling this circuit has nothing special going for it when compared to the 'usual perhaps more boring approach' but is better avoided as it is now.

W.r.t. Q2, you mean that the idle current is strongly determined by the whims of this PNP-device ? Other you mean another 'bad' mechanism ?

[/quote]

It smacks of one of those circuits that on a particular day with certain parts and a prevailing breeze, had some special magic to the ears of the designer.

You have two 'bad' mechanisms at play: 1) the absence of emitter resistors leading to thermal runaway limited only by the max current that can come out of the lower transistor and the proximity of the bias diodes, and 2) the positive tempco of beta setting that latter max current. As PRR points out the drive capability of the lower device is sorely limited---there could be cases where much of that current will come from the op amp through that 100 ohm resistor. But they are claiming "rich" bias---so that implies that the 2955 has high beta.

Note as well that what bit of pulldown current there is diminishes as you swing negative, coming as it does from only that resistor.

Or---wild thought---maybe those reversed diodes weren't reversed! Perhaps the whole of the output current is set by 2955 beta times the 150k R2's current, and the diodes get forward-biased on big swings to actively drive the PNP. At higher frequencies the PNP gets drive through the R-C network C2-R4, so this isn't too far-fetched, although still not the way I would go.

Lets be careful here, this is the exact reason we had a visit from a well respected marketing type. Not for ripping off and posting a copywrighted schemo, but for ripping off and posting a messed up schemo, which gave a cool tube equipment company a bad rep. To a few eyes at least.

[quote author="CJ"]Lets be careful here, this is the exact reason we had a visit from a well respected marketing type. Not for ripping off and posting a copywrighted schemo, but for ripping off and posting a messed up schemo, which gave a cool tube equipment company a bad rep. To a few eyes at least.[/quote]

I can understand how someone would feel. I believe it was Schoenberg who said "My music is not modern, it is just badly played." One could imagine someone complaining that their designs were great, just improperly reproduced.

This could lead to all manufacturer documentation accompanied by an intensely detailed Principles of Operation text.

[quote author="PRR"]It is hard to believe the plan is correct as drawn. If so, then it seems to be tuned for negative clipping "sound", since 150K+2N2955 will not give much pull-down in 100Ω.

National has a DSL chip re-rated for headphones that will drive your 100Ω with authority and transparency.

The smaller OverTure loudspeaker amp chips ("Gainclone") will work.[/quote]

Thanks for the suggestions. I do realize there's indeed a much simpler way than all those separate components. People can be impatient though: they want to solder now with common parts i.s.o. ordering specific parts & having to wait a bit. It's just the idea of course, since those few days won't matter when looking at how long it'll take to get the rest of the complete EQ-box done... :wink:

Many headphone drivers will work. The Tori is a power-pig but simple and should be faultless.

Click to expand...

It sure looked attractive. If all those parts I'm now using don't do the trick I'll be trying that one next.

Since this is DC-coupled into high-H inductors, you may need lower DC offset than we accept in other work. For studio work, you may be fine selecting a low-offset chip and adding a trimmer. For outdoor gigs, you should probably use a servo with a very low offset chip.

Click to expand...

I happened to have a few OP07 opamps around, very low offset. These seem fine for the servo.
Any ballpark-figure I should aim for ? I guess I can keep the VDC as presented to the EQ below say 2mV.

Thanks,

Peter

[quote author="CJ"]A transformer would be nice here.[/quote]

At the input instead of all this active stuff ?
The idea was to properly drive the Pultec-EQ-section at all settings a
while it's not always known the driving source is up to that task. So to isolate those two things an active thing looked OK.

Bye,

Peter

[quote author="bcarso"]
It smacks of one of those circuits that on a particular day with certain parts and a prevailing breeze, had some special magic to the ears of the designer.

You have two 'bad' mechanisms at play: 1) the absence of emitter resistors leading to thermal runaway limited only by the max current that can come out of the lower transistor and the proximity of the bias diodes, and 2) the positive tempco of beta setting that latter max current. As PRR points out the drive capability of the lower device is sorely limited---there could be cases where much of that current will come from the op amp through that 100 ohm resistor. But they are claiming "rich" bias---so that implies that the 2955 has high beta.

Note as well that what bit of pulldown current there is diminishes as you swing negative, coming as it does from only that resistor.
[/quote]Maybe that 150k is in reality much lower ? Would at least improve the situation you mentioned a bit.

Or---wild thought---maybe those reversed diodes weren't reversed!
Perhaps the whole of the output current is set by 2955 beta times the 150k R2's current, and the diodes get forward-biased on big swings to actively drive the PNP. At higher frequencies the PNP gets drive through the R-C network C2-R4, so this isn't too far-fetched, although still not the way I would go.

Click to expand...

In that mysterious schematic (I wish I still knew from who I got it !) they are indeed both drawn 'k' up, 'a' down... but it looks so... hmm, who knows.
The circuit as originally (re-)drawn has at least a few 'value-imperfections' (10k & 0.47uF giving imho too high a corner frequency for instance) so I don't know what to think of its total accuracy (sorry, whoever you are :wink

Bye,

Peter

[quote author="CJ"]Lets be careful here, this is the exact reason we had a visit from a well respected marketing type. Not for ripping off and posting a copywrighted schemo, but for ripping off and posting a messed up schemo, which gave a cool tube equipment company a bad rep. To a few eyes at least.[/quote]
I see what you mean - I hope it's all still OK.
Point is, the appreciation for this circuit stands or falls with the accurateness of the 'redraw'.

Could that kind person say 'hi' here (& have another look under the hood perhaps ?) Or other people having access that box ?

Bye,

Peter

Made some changes to the circuits from the Lab-theads of yesterday.
Since most is going on here I'll add it here.

(Please click the pic for visibility - or there's also the pdf)


http://home.hetnet.nl/~chickennerdpig/FILES/Pultec/driver_04.pdf

So most-things-'M' are gone, it's in fact now starting all over from the
Gyraf headphone driver and simply adding a DC-servo and a balanced input.
As it is now it could use some more biasing perhaps. BJTs now at 40mA.

I'll be breadboarding this version, I'll post more. But I expect it'll just work, no mysteries.
Out of curiousity, that 'M'-circuit might follow after that for a comparison.

Have a good weekend,

Peter

> Maybe that 150k is in reality much lower?

It would make more sense. At 150K, and assuming 2N2955 Hfe is 20-100, the large-signal output resistance is 1.5K-7K, quite large compared to a 100Ω load. But with all the other suspected transcription flaws, and the fact that this company's products are more than the sum of the parts, so they hate to see schematics in public, I'd rather drop it. If you want to copy another Man's product, you need more than the schematic, you need the parts-selection, the build-quality, the critical inspection, and the karma (I believe there is good karma on that Man's assembly line).

> it'll just work, no mysteries.

Well, it will work, but there are issues.

At a glance: the 5532 is asked to drive R1||R2 through C2 C7 and the AC-grounded supply rails. 235Ω, woof. If the 5532 could drive that, two in parallel could do the 100Ω simpler than all that other stuff. (In fact it would take 6 to 10 sections of 5532 to drive 100Ω well.)

In fact it is not necessary to bootstrap to both supply rails. Let the opamp drive the top base, and use diodes and resistor/bootstrap/CCS the bottom base.

Idle current of 40mA is not bad, but is neither Class A nor Class B for large swing into 100Ω. It is class A up to 8V peak.

The input impedance of these one-opamp "balanced" inputs is never balanced, though it never seems to matter in studio work.

> VDC as presented to the EQ below say 2mV.

Find out how much DC your inductors can stand, and their DCR, and how much of the amp offset voltage reaches them. NYD showed some transformers that seemed to be drooping at just 1mA. DCR was IIRC like 200Ω. Hmmm: 1mA*200Ω is 200mV, so 2mV ought to be negigible. But I'm not sure how to scale this to treble coils, you better check the actual parts.

I haven't studied the slew of messages/threads relating to this in detail. But am I correct in understanding that all this circuitry is just to drive a Pultec filter section? :shock:

I know that hashing out these circuits is fun, and I'm not trying to piss on your parade, but... Honestly, I don't think very low-Z, high-level drive is what you want to feed a Pultec, not if you want a "classic" sound. Consider that the sources fed into a Pultec in the old days would be of fairly high source impedance, 600 ohms, maybe as low as 100 Ohms. With a 600-ohm source, 6dB was being lost at the input under "best case" control settings. Drive that filter with a high-level amp having a very low source Z and your inductors will be saturating like mad.

If you're really opposed to the idea of a stepdown transformer, for whatever reason, and you want to isolate the signal source from the filter, how about a "taper pad?"

Or, a simple active solution: a 5532 or 5534 set for a gain of 6dB, with a build-out resistor of 590 ohms on the output.

I"m a "keep it simple" kinda guy, myself :wink:

But am I correct in understanding that all this circuitry is just to drive a Pultec filter section?

Click to expand...

I don't think so--it's for driving a massive 150 ohm passive EQ.

A few thoughts on the new revision:
* Add a 10 ohm in series with R3 and a 100 pF (or a 47 pF to correct for C1 :?: ) across R3 to improve CMRR.
* I would replace C8 with two 220 pF caps going to ground (actually chassis/pin 1)--most RFI is CM and as shown this filter is acting for differential signals only and at a pretty high frequency.
* I agree with PRR that the bootstrapping is probably overkill--for this application, two red LEDs with one 6k8 to the negative rail could work very well. The high forward voltage drop makes you use large emitter resistors (about as shown now) which is good for temperature stability (they could negatively affect class AB distortion so make sure you get enough current for class A). The temperature stability is improved by the matching tempco of the red LEDs.
* Personally I prefer servo opamps with input bias current compensation (I use OPA177); that saves me R15/C9. But nothing wrong as shown.

Samuel

[quote author="NewYorkDave"]I don't think very low-Z, high-level drive is what you want to feed a Pultec, not if you want a "classic" sound[/quote]

The manual of this thing (Ma$$ive Pa$$ive) says that the "trick is to make the coils saturate just the right way" .. this is probably what they're after.

I don't like coil saturation much myself.. but maybe it's a matter of having the correct type of coil with just enough drive. A Toko inductor is probably not the right type of a coil. :razz: :razz: :razz: