Schematic analysis of the Marshall Valvestate 8008

[Roberto]

Dear all,

I've just receipt a broken Marshall 8008 paid 25 euros and I would like to tweak it to find how can I like it, but solid state power amp are not exactly my main field of knowledge.
Please correct me and/or integrate what I'm missing.

This is the schematic: http://www.drtube.com/schematics/marshall/8008.pdf

- C2 R2 and R1 make a highpass at 18 Hz;
- C1 and R1 make a lowpass at 10260 Hz;
- C3 and R3 make a lowpass at 4800 Hz and set the gain together with previous network;
- C13 and R9 make a hifhpass shelving at 7200 Hz together with the TR7 and TR16 network;
- D2 D3 R27 are a safety system for TR7 and TR16;
- TR7 and TR16 drive the following HT supplyed power stage by the dual 15 V supply;
- R45 is a sort of feedback, I guess;
- TR2 and TR10 drive the power stage;
- R24 R25 and TR5 are an amplified diode;
- TR1 and TR14 are the output transistors;
- R13 and TR6 plus R34 and TR15 seems something like a compressor, a variable voltage divider that limits the pilotage of the power transistors as they increase the voltage swing;
- C22 and R59 are the zobel filter;

But then...
- C19 R57 and R58... the cutoff frequency is 2 Hz, so what's its purpose? Why not just a resistor?
- R55 is in parallel with R57 and R58, again why?
- how the CFB and VFB works exactly?

Can anyone please help me?
I would like to adjust the response of this poweramp to my tastes, and with tube amps I know how to do it, while with this amp there are few points I'm missing.

Thank you all in advance.
Roberto

[Raybob]

You need to ask those questions to the Marshall company or the person that designed that amp. I seriously doubt there is anyone posting on this forum that would ever design such an amplifier, nor know the reasoning of the questions you ask.

[Roberto]

Thanks for the tip!
I've just sent them an email!

[Mike Sulzer]

This amp has two output modes: voltage (low output impedance) and current (high output impedance). The two are achieved by different kinds of feedback to the input op amp, either the voltage output of the amp, or the voltage across a small resistor in the low side of the circuit, which is proportional to current. When in the current mode, you want it to actually be voltage mode at very low frequencies and dc so that the amp has a stable operating point. The components that you mention are part of the circuit that allows that to happen.

I have not worked out the details, but I suspect that it is not a perfect current source (close to infinite impedance), but similar to the output impedance of some reference tube amp. Tube amps tend to have higher output impedances than solid state, and this affects the frequency response because the speaker impedance is a strong function of frequency. If you use several different kinds of speakers with different impedances vs. frequency with an amp, then the differences that you hear with a tube amp will not be consistent with an ss amp run in the voltage mode.

Dear all,

I've just receipt a broken Marshall 8008 paid 25 euros and I would like to tweak it to find how can I like it, but solid state power amp are not exactly my main field of knowledge.
Please correct me and/or integrate what I'm missing.

This is the schematic: http://www.drtube.com/schematics/marshall/8008.pdf

- C2 R2 and R1 make a highpass at 18 Hz;
- C1 and R1 make a lowpass at 10260 Hz;
- C3 and R3 make a lowpass at 4800 Hz and set the gain together with previous network;
- C13 and R9 make a hifhpass shelving at 7200 Hz together with the TR7 and TR16 network;
- D2 D3 R27 are a safety system for TR7 and TR16;
- TR7 and TR16 drive the following HT supplyed power stage by the dual 15 V supply;
- R45 is a sort of feedback, I guess;
- TR2 and TR10 drive the power stage;
- R24 R25 and TR5 are an amplified diode;
- TR1 and TR14 are the output transistors;
- R13 and TR6 plus R34 and TR15 seems something like a compressor, a variable voltage divider that limits the pilotage of the power transistors as they increase the voltage swing;
- C22 and R59 are the zobel filter;

But then...
- C19 R57 and R58... the cutoff frequency is 2 Hz, so what's its purpose? Why not just a resistor?
- R55 is in parallel with R57 and R58, again why?
- how the CFB and VFB works exactly?

Can anyone please help me?
I would like to adjust the response of this poweramp to my tastes, and with tube amps I know how to do it, while with this amp there are few points I'm missing.

Thank you all in advance.
Roberto

[Mike Sulzer]

You need to ask those questions to the Marshall company or the person that designed that amp. I seriously doubt there is anyone posting on this forum that would ever design such an amplifier, nor know the reasoning of the questions you ask.

I am sure that there are at least several people who post here who could answer those questions.

[teemuk]

- D2 D3 R27 are a safety system for TR7 and TR16;

They establishes bias voltage difference between bases of TR7 and TR16.

- R45 is a sort of feedback, I guess;

Looks like local negative feedback for the amp section following opamp input stage.

- R13 and TR6 plus R34 and TR15 seems something like a compressor, a variable voltage divider that limits the pilotage of the power transistors as they increase the voltage swing;

Sort of. "VI Limiter" more commonly. Base circuits of the transistors monitor output transistor current, and start to divert base current in case excessive output transistor emitter current is detected. Crudely.

- C19 R57 and R58... the cutoff frequency is 2 Hz, so what's its purpose? Why not just a resistor?

Resistor and an RC filter are not equivalent circuits throughout the frequency spectrum. I'm not exactly sure what the purpose of this filter is but I have seen it in some Fender PA amps too. I wouldn't immediately categorize it as something you can simply substitute with a resistor.

- R55 is in parallel with R57 and R58, again why?

That's the (voltage) negative feedback path; That's not just bunch of resistors in parallel, there's a resistor in parallel with an RC filter circuit. Different NFB characteristics for AC vs. DC, and I think there's some "shelving" introduced to the filter function.

- how the CFB and VFB works exactly?

Like in most cases, there's negative (voltage) feedback directed from amplifier's output to its inverting input. (Yes, it shows an non-inverting input but if you look at entire PA circuit it inverts again so one needs to reverse the terminal functions of the input opamp). Anyway, the negative voltage feedback is taken directly from amp's output, at which terminal the amp tries to keep voltage rather independent of load current fluctuations... hence "voltage feedback".

Amp's gain, partially, is adjusted to proper ratio by attenuating some of the (voltage) negative feedback signal. A divider circuit is needed. Series signal path of the divider is through R55 combined with filter, the "shunt" signal path is through R56 and AC coupled with C23. Finally the shunt path terminates primarily through very low resistance of R60.

Note, the aforementioned naturally applies only with the switch in "current" position. In "voltage" position the feedback "shunt" path is through R61 instead.

Practically ALL load current also flows through R60, that is after flowing through the reactive speaker load. We have a voltage divider, in which the upper end is the reactive impedance of the speaker load and bottom end the "fixed" resistance of R60. The ratio of these components is held small, which allows the divider to work effectively. Now we have load current modulating the signal voltage at terminal connecting loudspeaker and R60. This signal modulation, due to load current, mixes in with main voltage feedback because both terminate through same shunt signal path. On contrary to negative voltage feedback, negative current feedback artificially increases amplifier's output impedance altering its damping characteristics. Effectively amplifier's gain becomes dependent on load impedance.

Everything else is just "voicing", tone shaping, with the associated filter circuits.

[Roberto]

Dear all,

thank you for your explanation, I will go more in detail tomorrow, as now here it's a few minuts past midnight.
This evening I've done some eq-sweep simulations of the circuit, enjoy them in attachment.

Valvestate CFB C1 100p 470p 2n2.pdf
Valvestate CFB C19 10u 100u 1m.pdf
Valvestate CFB C2 47n 220n 1u.pdf
Valvestate CFB C23 1u 10u 100u.pdf
Valvestate CFB C3 47p 100p 470p.pdf
Valvestate CFB C13 4n7 10n 47n.pdf

[Dave H]

R17, D2, D3, R27, R18 set the bias voltage for TR7, TR16. R28, R44 set the bias current through TR7 and TR16. This bias current then sets the bias voltage across R16 and R41 for TR2 and TR10, their bias current being set by R15 and R40.

R24, R25, TR5 set the bias voltage for the output stage (TR1, TR14) and R53, R54 set the bias current.

TR6, TR15, R26, R43, R14, R42, R53, R54 provide output stage short circuit protection.

I think R2, R3 and R9, R45 set the gain of the op-amp and output stage to limit the open loop gain of the whole circuit so that it doesn't become unstable when global negative feedback is applied.

[J M Fahey]

You asked this exact same question in The Amp Garage , I answered there in full, but you seem not to have read it, so I paste it here:

"roberto" Dear all,

I've just receipt a broken Marshall 8008 paid 25 euros and I would like to tweak it to find how can I like it,

Not much to tweak, sorry.

- C2 R2 and R1 make a highpass at 18 Hz;

yes

- C1 and R1 make a lowpass at 10260 Hz;

no, the lowpass is C2 + R2 + whatever VR1 is set to, max 2k5 , so your lowpass is roughly 10k into 470pF , do the math (roughly 3X what you calculated early)
And it's not there for frequency compensation but to attenuate RF interference.

- C3 and R3 make a lowpass at 4800 Hz and set the gain together with previous network;

Yes to gain, that lowpass maybe meant to smooth a buzzy signal .

- C13 and R9 make a hifhpass shelving at 7200 Hz together with the TR7 and TR16 network;
R45 and R9 make a local feedback network, which allows the output stage to have a gain of ~4:1
What for?

The Op Amp output can not swing much more than +/-15V , and in fact 2 or 3 V less than that on each side, while speaker out must be able to swing +/-42 V , so output stage needs some local voltage gain, around 4X .

- D2 D3 R27 are a safety system for TR7 and TR16;

No, D2 D3 R27 bias Tr7 Tr16

- TR7 and TR16 drive the following HT supplyed power stage by the dual 15 V supply;

Sort of.
Tr7 Tr16 are called "level shifters" , whatever signal appearing at their emitters will also appear at their collectors , but "shifted" some 40 V , the DCV difference between the center rail and Tr2 Tr10 base, which sit close to power rails
Otherwise you would need a driver transformer to drive them from the Op Amp output.

- R45 is a sort of feedback, I guess;
- TR2 and TR10 drive the power stage;
- R24 R25 and TR5 are an amplified diode;
- TR1 and TR14 are the output transistors;

yes

- R13 and TR6 plus R34 and TR15 seems something like a compressor, a variable voltage divider that limits the pilotage of the power transistors as they increase the voltage swing;

Almost.
They are the short circuit protection.
Normally turned off, when current through R53 and R54 rise above ~8A they turn on ad limit/clamp/short base drive to power transistors.

- C22 and R59 are the zobel filter;

yes

But then...
- C19 R57 and R58... the cutoff frequency is 2 Hz, so what's its purpose? Why not just a resistor?

Also include C23 in the question.
Although 2 Hz is very low, it is not DC.
We do not want the amplifier to have DC gain because any small DC error at the input, called offset, will appear multiplied 20X or 40X at the output, which is bad, both to the speaker and the transistors.

- R55 is in parallel with R57 and R58, again why?

Not exactly.
Signal can always go through R55 while the one branched through R57 R58 will get different frequencies and voltages injected through C19 and depending on setting of SW2 switch.
By the way, that weird NFB network is the base of the "Valvestate" sound.

- how the CFB and VFB works exactly?

Can anyone please help me?

That merits another chapter in the book

I would like to adjust the response of this poweramp to my tastes, and with tube amps I know how to do it, while with this amp there are few points I'm missing.

Short anwer, you can't.
It's a very complex amplifier, very well designed, has been tweaked as much as is possible, anything you do will make it sound worse ... believe me.

Study a lot and design a preamp or distortion circuit or EQ or compressor or anything else you want *outside* this Power Amp, *then* feed that into it (and into great speakers) .

FWIW that's what Billy Gibbons does (and his sound is killer): a huge Rack, his preamp varies (although it's often a Marshall JMP1) , driving a few ...... Marshall 8008 power amps



I strongly suggest you download and read, first the introduction to Jack Darr's Guitar Amplifier Book (one chapter was already linked above) , google the other chapters which are available, of course best is to buy the full book (Amazon usually has a couple, sometimes used, worth every cent) and then follow with the best book on SS amps, period: Teemu Kyttala's http://www.thatraymond.com/downloads...ttala_v1.0.pdf
_________________
Design/Make/Service Musical stuff in Buenos Aires, Argentina, since 1969

[teemuk]

Roberto,

It looks like you are using a resistive load in your simulations. Substitute it with something like this...

..and I'll bet you'll understand more clearfully concepts like "damping factor" and "current feedback".

For reference, a graph of impedance characteristics of a genneric dynamic loudspeaker... looking at it, concept of "nominal" impedance is quite understandable...

[Roberto]

Muchas gracias Juan,
Voy leer el link que me inviaste y preguntarte mas informaciones.

Thank you Juan,
Your help has been highly appreciated, I will for sure read (in fact I've already started to do it) te link you sent me.
I have to say that I've a different 8008 compared to the ones shown in that ZZ-Rack: just two volumes in front and the CFB/VFB switch on the back.

I tried to reply you on ampgarage, but it has become impossible with my ipad. I hope they'll solve this issue shortly, because it's a pity.

[Roberto]

Thanks teemuk,

Catched. Yes, as a first simulation I removed the VI limiters and simplified the load using just a resistor.
I will update the schematic with the right load, while keeping out the VI limiters by now (I've a student-licensed simulator that would go out of components if VIs will be implemented).

Let me update the files when ready.

[Roberto]

Thanks, done the simulation and got the difference between the two.

As for the issue, there were two BDV64 and BDV65 shorted.
I've ordered two pairs and in the meanwhile I'll check the other voltages, to control they are fine.

[Enzo]

You can directly substitute the common and inexpensive TIP142/147 pair in place of those BDVs.

[Roberto]

Thank you for the tip Enzo,
I've seen the compatibility while searching for substitutes, but then I've found some cheaper BDV pairs on ebay and bought them.