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Old 27th January 2008, 03:29 PM   #1
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Question My first amp design - comments

First off all, I must say that English is not my native language, so you must excuse me for any grammar and spelling errors.

In past several months I have spent a lot of time in designing an amplifier for home use. I was inspired by a book named “Audio Power Amplifier Design Handbook” written by Douglas Self. Some parts of that book can be found on his site. After many hours... no months, of circuit simulation using Multisim 7 and freeware SwCAD III and later Multisim 10, I have come up with a nice, simple circuit, with more than enough power for home use. The circuit is presented at the bottom. It would be nice if someone could give me some guidelines or advices to improve the circuit. If you think that there are (or could be) some errors present in the circuit please give me a hint. In a month or two I would begun with PCB design.

Below is given simulated performace:

THD (1W, 8 ohm) 1kHz: 0,001% 20kHz: 0,002%
THD (16W, 8 ohm) 1kHz 0,001% 20kHz: 0,004%
THD (60W, 8 ohm) 1kHz 0,001% 20kHz: 0,004%
THD (95W, 8 ohm) 1kHz: 0,008 20kHz: 0,059%

Transistor Q15 is mounted on Q14 as a sensor for bias voltage. Small signal transistors used are 2SC2240 and 2SA970. Output devices are ONsemi products: MJL21193/21194.

If you have any question I would be happy to answer them.

Thanks for your comments and advices in advance.
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Old 27th January 2008, 04:24 PM   #2
teemuk is offline teemuk  Finland
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Quickly viewed it looks good to me. I would suggest that you run the simulations using a circuit model of a realistic supply (one rectified from AC and having some series resistance at the power source). This often reveals some weaknesses in the design. I would also “isolate” the supply rails of the driver and VAS stages from the main supply rails with a RC filter. Likely a 100R and 100 uF would be all right.

I don’t know if there’s much help from the current limiter Q7 unless you current limit the output stage as well. In fact, at those power levels I suggest that you add this feature too. Also, connect two “anti-parallel” diodes from the base of Q1 to base of Q2. This will tie the inputs to about the same voltage potential even during clipping or slewing and thus protect the input from reverse biasing.

Just my
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Old 27th January 2008, 04:50 PM   #3
Donk is offline Donk  Europe
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Hi,

Few comments:

- R21, R22, R28, R29 seems way too low for me. I'd use 0.33r there, then bias for 25...26mV Vre would be lower too, 80mA instead the 210mA (35W idle dissipation!). With that higher calue thermal stability would be better.
- R17 is too low, use R14 devided by three. Then change R17 to 22r.
- isn't R5, R6's value too ow? I'd switch to 47r or 68r (100...150mV voltage drop on them).
- for R3 and R4 emmiter degenerating resistors I prefer R=26/I(mA) > 12r.
- voltage reference for CCSs (formed by Q6) MUST be on the LTP's side!
- MJE340/350 aren't the best devices for VAS. Use 2SA1360 or similar (high Ft, excellent linearity, low Co) if you can get them.
- I'd change R16 to 1k for 1mA collector current for Q8.
- experiment with square wave oscillator and with an o'scope for the best value for C7, try 100n...1u
- you'd need a "fine tune" for C15, 10pF...100pF. Same for dominant capacitor ("C")!
- about R24: most designers prefer 10r, but I think 2.2r or 4.7r is enough there. For L1 1uH is enough.
- Teemuk, no need for isolation. VAS's and LTP's current topology gives excellent PSRR.
- what about adding a simpe OP short-circuit protection?
- rail fuses? 5A fast blow w'd do the job.
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Old 27th January 2008, 09:51 PM   #4
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Hi Donk, I agree with all your suggestions, but I think Teemuk is right. I always use separate power supplies for the input and vas stages (one for the 2 (or 7...) input and vas stages, and one for each final stage). Infinite PSRR is the best ! Perfect ground wiring is an absolute necessity...

PL.
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Old 28th January 2008, 07:24 AM   #5
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I made this amp strictly by the book, this is almost a copy of example amp given in the book. I have omitted protection circuit (simple single slope or multislope VI limiter) from schematic for the sake of simplicity. Because of that Q7 was added in schematic.

Teemuk, I will add anti-parallel diodes to Q1 and Q2. Good thinking! Thanks

Donk:

R21, R22, R28, R29 were increased to 0,33 (I too tought that this was too low, but the book sad otherwise and Self in one of its amplifiers (class B) uses only 0,22ohm)
R17 changed to 22r (26mA current to switch Q7 protection, isn't this still to high?)
yet again, the book proposed 40-60mV as enough voltage drop on R5, R6 – changed to 68r (136mV)
I knew the R=(25 or 26)/I[mA] equation, it is even mentioned in his book, but he praised high value degeneration resistor as a way to linearise LTP. I was very doubtful if this is true, as no one (not majority) is using this high values.
Q6 in on LTP's side now
I can't find any spice models for 2SA1360/2SC3423, so I used MJE models. Didn't know of them until I saw PGP amplifier project found on this forum; will try to find them
R16 changed to 1k
C7: need more time to get the right value

Thank you Donk! It looks like that with these new values the amp will deliver less THD at the same output power level. 1kHz went to 0,000% (can't increase precision to 4 decimals). At 95W and 20kHz I can sometimes see some artifacts in output signal (pic below) and THD rises, so C, C15 and C7 needs some more tweaking.

About separating power supplies for LTP and VAS from the output stage; I also think that there is no need to do such a thing, remember, this will be a home amp and not a high end design. Maybe I will add RC filter only in negative power supply line to deal with the problem.

Once again, thank you everyone! You were very helpfull
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Old 28th January 2008, 08:11 AM   #6
jaycee is offline jaycee  United Kingdom
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I can recommend KSA1381/KSC3503 instead of MJE340/350. These are excellent transistors from Fairchild that are ideal for VAS use. They also have SPICE models.

Instead of 2SA970/2SC2240 there is also 2SA1015/2SC1815, and Fairchild second source these as well (as KSA/KSC). They have good SPICE models for their version too
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Old 28th January 2008, 12:21 PM   #7
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So, here is a new "improved" schematic not blindly based on Self's book

THD at 20kHz has dropped from 0,059% to about 0,02%, THD at 1kHz is barely measurable and sometimes shows 0,000%

Now I need to be sure that amp is absolutely stable and doesn't become an oscilator
so the capacitors C, C9 and C15 need to be fine tuned
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Old 28th January 2008, 12:21 PM   #8
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jaycee:

I can't buy those transistors here, but can I use spice models for KSA/KSC to simulate 2SA/2SC devices?
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Old 28th January 2008, 02:48 PM   #9
Donk is offline Donk  Europe
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For me it looks better now!

As Cdom first I'd try 100pF, and then down to 82...68...47pF see what happens related to stability of the whole amplifier. Maybe you should try two-pole compensation, formed by 100pF and 470pF and an 1k resistor.
As C7 I'd try 220nF foil-cap, then watch 10...20kHz square-wave and sine-wave response of the amp, first at low (100mW, 1W) and then at high (100w) amplitudes.
C15 maybe sholuld eliminated. I wouldn't use it if the amp is absolutely stable with that. It can cause oscillations too, see the member called MJL21193's "Patchwork" thread.

Maybe you should try Microcap as simulator, I don't trust Multisim (I've got MS8 and MS10) for amp design.

PS.: if you can get 2SC4793 and it's complementary PNP pair then use them, they're better then MJE15030/31, and aren't so expensive devices. I found that MJE's beta difference is way too large between NPN and PNP.
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Old 28th January 2008, 04:45 PM   #10
jaycee is offline jaycee  United Kingdom
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Yep, use the models for sim purposes It's a shame you cant get those transistors but if it can handle the fast, low capacitance VAS models, it'll handle the old MJE340/350!
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