A "Patchwork" Type Concept Amp

[Stormrider]

Here is an idea for a circuit i drew up. It's nothing new, just a few things I "patched" together. My own "patchwork" amp of sorts. The output stage is dual (?) CFP, which I saw a sketch of posted by Event Horizon in a thread recently. Input stage is mostly P101, and the VAS is generic. There will be an RC filter for the front-end, and bypass caps for the output stage; I just left them out to keep the diagram simple at first. I'm also thinking about adding VI limiting.

This is merely a concept drawing.

I haven't figured out LTspice yet, so I'm awaiting that to see if this will even work, and to figure out the component values. Some are already filled in with arbitrary values.

I'm looking for ideas, but take it easy on me. This is my first, albeit pseudo, design. If it looks good, I'll be looking for device suggestions for the frontend, VAS, pre drivers, and drivers. I'm thinking +/-45Vdc to +/-55Vdc rails with two pairs of MJ2119X outputs.

[sakis]

this type of output has been presented in the past in the forum by another greek member .... The person who designed this, claimed that topology with one driver per one output transitor had amazing drive ability and actually forced outputs to their best ... ( very good amp for wierd or watt asking loads )

in practice when he brought his amp to the shop it prooved that had very limmited bandwidth to preserve stability and other than that was an oscilation nightmare ...

not to mention a gozillion millers/snubers here and there to stop oscilations that actually managed up to a point but killed all amplifier sonics

so yes ...it looks like a very nice idea but the trick is to find ways and pcb to keep both stability and bandwidth while oscilation free ...

not an easy task ....

[h_a]

Well, if the CFPs really make problems you can always hack the board to emitter follower outputs

Have fun, Hannes

[AndrewT]

Hi,
excluding the CFP, which I cannot comment on, I'll start at the front.
Add a DC blocking cap. Select it to set the bass passband. All other bass limitations in the amplifier must be of lower frequency than this HP filter.
Change the MPSA42 in the LTP. You need high gain, high frequency, low Cob, adequate Vce0, flatish gain with Ic.
Add degeneration resistors to the current mirror.
Adjust the base current of the VAS to match the total base currents of the two mirror transistors. (it took me 40years and the help of GK to realise this).
Q3, 4 & 6 do not need to be the high Vce0 version. bc550c/560c would be better here. Similarly Vbe multiplier does not need medium power device. It is only used for the convenience of having a fixing hole in it.
Change the VAS transistor from a driver type to a more suitable device., eg. 2sc3423
Add D.Self's resistor to the Vbe multiplier. Move the driver base to the Vbe collector. Add in the extra resistor between the top of R11 and the base of q10. This is particularly useful when the VAS is not supplied with a constant current. The Vbe likes to see a constant current and Self's resistor increases the tolerance to small changes in Vbe current.
C3 & C5 must be adjusted pro rata with the DC blocking cap.

Change Q9 & Q10 to more suitable devices, these pre-drivers could be same spec as the VAS

Relabel the output resistors. Your .22r looks like 22r and should read either 0.22r (never omit the leading zero) or 0r22, then there is no possibility of confusion.
Connect the signal grounds together. RCA ground PCB input ground, NFB ground, RIn resistor ground, RF filter cap ground.
Where should r7 ground go?
Taken to the signal ground and it adds a dirty current to the clean side.
Take it to the decoupling/Zobel ground and it gets contaminated by the horrible pulsed currents around there.
Taken to the +ve supply rail and decoupled back to the -ve supply rail? Is that too elaborate?

Do not take the NFB tapping to R9 from the output resistors. Take it from the single line out to feed the Zobel circuit.

read thread
http://www.diyaudio.com/forums/solid...t2136099<br />

[Stormrider]

2SC2240 look better for the LTP.

As you said, the Vbe multiplier is BD139 so it could be securely fastened to the same heatsink as the drivers.

As of now, the 2SC3423/2SA1360 is available at Newark. I'll use them for the VAS/pre-drivers (depending on the output stage I choose).

I really need to get Self's book. The added resistor makes sense. What about the cap across the bias circuit? It's C4, 47uF in the Blameless schematic on his website. He shows it going across the entire bias circuit, whereas Leach shows one going just across C-E of the bias transistor. What's the difference, and what does it do?

How is Fc calculated for C3, C5? For C3 is it as simple as 1/2(pi)(R8)(C3) ?

R7 seems to be a conundrum. I've seen people take it back to the signal, clean gnd, and others take it to the dirty gnd. Since it's in the input stage I think taking it to the clean gnd would be better. I'm not sure what you mean about taking it to +V, then decoupling it -V. I would be interested to know your solution, elaborate or not.

As for the output stage, it seems this scheme is not well thought of. I like CFP, in theory, so would it be better to stick with the more basic/common scheme using a single driver pair and two output pairs? Or, just revert back to triple EF?

[sakis]

cfp has more "musical approach " while efp tends to be more punchy and acurate ..This apply if bootstrap exists

but still ...dont give up that easy ...try it ...it might work but you have to go farther ...simulation will not be enough

[Stormrider]

I think I have everything you suggested taken care of Andrew. I also changed the output stage to basic CFP, still with pre-drivers.

Should a "speed-up" cap be added across R15? And, should there be a resistor from the wiper of the bias pot to the base of Q8?

For bias tracking, does Q8 need to be in contact with the pre-drivers (Q9,Q10) and the main drivers (Q11,Q13) or just the main drivers like a normal CFP?

[AndrewT]

R27 is in the wrong place. You moved the base of pre-driver to Vbe collector, But you also moved the VAS collector from the top of the bias resistor string. Put it back and then R27 is oriented horizontally between the resistor string and the Vbe.

Hold a constant voltage Vce across the Vbe.
trace through the output circuit and find which Vbe junctions are fed by that voltage. You should find series combination of Vbeq10+Vbeq11+r20+r21+Vbeq13+Vbeq9.
If the voltage across every one of these Vbe were fixed, then you can calculate the voltage across the resistors and that will determine the bias current and bias power.
Now change the temperature of any one of those 4 Vbe junctions. Will the current through the resistors change?
Change the temperature of all four Vbe junctions.
Do you think you should apply temperature compensation for pre-drivers or drivers?

The new routing of the NFB clarifies the diagram, you can aid this by re-locating C5 to the left of r13/r14 and then a short line to the NFB route. How about removing that long route and simply labeling the unterminated end with the same OUTPUT label so that the software knows they are connected.

[Stormrider]

It looks like if any of the four junctions shift the bias will shift, so all four should be in contact with Q8, or atleast one side (Q10/Q11 or Q9/Q13).

Corrected:

[event horizon]

Hi Stormrider, i couldn't resist dropping in when i saw my name mentioned

As has been pointed out, don't give up on the first type of double CFP you mentioned yet. I'm sure a lot of problems are caused by using what i call "fast" output transistors on the output stage. When D Self built most of his CFP stages he used Ft 3Mhz devices (as i have done in the past), not transistors with an Ft of 30+ Mhz. As you are planning on using MJ21193/4 you should have less trouble (Ft 4MHz).

By the way, you posted just before me & from what i can gather you might want to know where to clamp the bias transistor? Make sure that only the output MJs are on the main heatsink. Use a smaller heatsink which is seperate from the main one for the drivers (Q11/13) & clamp it on there. You'll only need something like a sheet of 1/8" alloy of about 6" x 2". Mount the drivers either side of the bias transistor & as close as possible The predrivers will be seeing so little current they aren't worth including in the bias loop.

E2A:- Btw, the second CFP type with a single driver & two outputs most definately works & i never had problems that even required attention. Don't forget that you might need a small low ohmic resistor between the driver emitters & the resistor on the collector of the outputs for stability. Never needed any myself & that was biased into class A where things could possibly be more problematic.

Good luck & have fun, i have had years of it Oh, you'll get to see me build some stuff soon enough i hope lol.

Bests, Mark.