Randy Slone's "Fig 11.4" (Self's "Blameless"?): PCB layout

[tcpip]

I've been trying to see if I could make a PCB layout for Randy Slone's "Fig 11.4" from his book "High Power Audio Amp Construction Manual." I'm told that his Fig 11.4 is an almost exact replica of Doug Self's "Blameless", with the addition of one or two small improvements.

I took this "Fig 11.4", and attempted to fit everything into one Eagle-Lite PCB (3.2" x 4"), and the result is shown below. I also added a few small "featurettes", based on my readings and on discussions in the "Fig 11.6" thread here. These include a diode and RC filter on each supply rail to shield the VAS and input stages from current sourcing and sinking in the OPS, and an input signal conditioning circuit. Thought I'd share all this and see what you guys think:





I have almost zero experience in designing power amps or power amp PCBs, hence all help I get is badly needed.

[pinkmouse]

Sorry, but there are lots of things wrong with this board, but two obvious ones for starters, Q7, the bias trannie, should be on the same heatsink as the output devices for stable operation, and the feedback path is far too confused.

I appreciate how difficult it is to get going with this, I've been there. But you need to block out your layout better, try and roughly follow the layout of the schematic, rather that just putting parts in where they fit on the board. Follow the logical signal path, and arrange components around it.

[tcpip]

Originally posted by pinkmouse
Sorry, but there are lots of things wrong with this board, but two obvious ones for starters, Q7, the bias trannie, should be on the same heatsink as the output devices for stable operation, and the feedback path is far too confused.

Can you please elaborate? I thought thermal coupling of the Vbias transistor with one of the pre-driver transistors is good enough in this case, because it's a CF OPS. I thought coupling with the OPS devices themselves is a strict necessity only in the case of BJT EF OPS.

And the feedback path seems to be a straight, fat track coming from the top of the board straight down to C6, R10, R11, to the base of Q2. Can it be any straighter than this in a congested PCB?

When I appear to contradict you, please don't misunderstand my intentions as an attempt to show how smart I am... I'm just trying to learn as much as I can from you.

I appreciate how difficult it is to get going with this, I've been there. But you need to block out your layout better, try and roughly follow the layout of the schematic, rather that just putting parts in where they fit on the board. Follow the logical signal path, and arrange components around it.

I thought I'd done that, in fact. The input stage is lower down in the board, the VAS comes after that, roughly, and the OPS is at the back. And I've tried to place devices "logically close" to their neighbours, so that track lengths are on the whole as short as I could get them.

I think I'll need more details.

[jaycee]

For CFP outputs (Sziklai) the VBE multiplier should track the driver's thermally, not the outputs.

Beware when using the CFP output. These must be well laid out or they will oscillate. If this is your first amp that you're designing youreself, stick to using an EF output stage. Personally I prefer the triple Darlington but that is just me

Why use a big 5W resistor for the output resistor/inductor ? A good 2W device is fine here, and you can wind the inductor around it.

For a low power amp (you'll only get about 100W with 3821/1302 outputs), you could use 3W collector resistors. Use vertical mounting to make it more compact.

Another change, put a 1K resistor in the base of Q8. Otherwise, if the VAS saturates due to clipping (such as the protection circuits activating) it will pull all the current from the voltage reference, and screw up the current source for the LTP. The result is usually rail sticking or oscillation. Better still... use a seperate current source.

[tcpip]

Originally posted by jaycee
For CFP outputs (Sziklai) the VBE multiplier should track the driver's thermally, not the outputs.

Thanks. This is what I'd read in Slone's instructions too.

BTW, my reading is limited to a little bit of Self and two complete books of Slone. I've also read online Web pages on the JLH and of course, some of these threads here.

Beware when using the CFP output. These must be well laid out or they will oscillate.

Slone says the CF oscillation tendency can be largely taken care of by using the appropriate driver devices, and since this is his design, I'm hoping he's made it stable enough for it to be forgiving of a novice PCB design. Anyway, I'm hoping to build it and see where it leads. I'm treating this as a learning experiment, not a world-beating amp.

Why use a big 5W resistor for the output resistor/inductor ? A good 2W device is fine here, and you can wind the inductor around it.

Actually that 5W resistor's part is supposed to be the size of my final inductor. Basically, I'm hoping I'll put a 2W resistor inside it, and wind the copper around it, and it'll finally come out as thick as the 5W resistor's layout.

For a low power amp (you'll only get about 100W with 3821/1302 outputs), you could use 3W collector resistors. Use vertical mounting to make it more compact.

If you say 3W is adequate, I'll use 5W, because I'm not very confident about the heat dissipation ratings of the resistors I get here (I'm buying from the local Bombay market). If I got a good international brand of resistors, I'd risk going by tighter ratings. I've been told by senior constructors that these resistors sometimes give up (i.e. get destroyed) due to heating.

Another change, put a 1K resistor in the base of Q8. Otherwise, if the VAS saturates due to clipping (such as the protection circuits activating) it will pull all the current from the voltage reference, and screw up the current source for the LTP. The result is usually rail sticking or oscillation. Better still... use a seperate current source.

Aha... the base of Q8.

If you don't make a face or fall asleep, I want to tell you about my story of that resistor.

Basically, I started working on the Fig 11.4 (BJT CF OPS) and Fig 11.6 (L-MOSFET hybrid CF OPS) amps in parallel. Since Slone says they are identical in all respects other than OPS, I entered the schematic from just the MOSFET circuit, and made two copies. I altered the OPS of one of them, and that became BJT OPS "Blameless". I'm still editing both in parallel, keeping them in sync.

Much later, while doing a detailed comparison of my BJT schematic with the one in the book, I realised that Slone had made changes in various little bits of the VAS and input stage after all. Some of those changes I could figure out, hence I made them consistent across both. For instance, in the resistors behind the Vbias transistor, he had just one resistor in the upper leg and one pot in lower leg for the MOSFET design, but he had a second resistor in series with the pot for the BJT design. I put that second resistor in for both circuits... it won't do any harm and will increase both sensitivity and safety. (AndrewT helped me a lot in that other thread on the MOSFET design). In the process, I learned quite a bit about how the Vbias setup operates.

One place where there was a difference was this resistor in the base of Q8. In the BJT design, he's put base resistors for both Q8 and Q3 (IIRC they're 1K each). In the MOSFET design, they're not there. Since I'd started copying with the MOSFET schematic, they're not present here too. I was discussing this only yesterday with one of my senior-designer friends and we couldn't figure out the purpose of the resistors. Now, with your explanation, I know. I'll put them in.

Should I put them in for the MOSFET design too?

About a separate current source, even AndrewT had said this is better. I am running too tight on space, so I'm dragging my feet. It'll require an extra transistor, three or four resistors and an electrolytic cap.

[tcpip]

AndrewT was suggesting that 47 for R22 is beginning the VAS protection triggering at only twice the CCS rating. He thought a larger latitude would be better to avoid protection-induced distortions on transients... maybe 22 instead of 47. Since I have no clue about these things, and his suggestions made sense, I thought I could always proceed with the topology and PCB now, fix the values later.

What do you think about that resistor?

Incidentally, it's 47 for both the MOSFET circuit and this one.

[jaycee]

Your resistor/inductor won't be that big, i think. On my own circuits it's only 12.7mm long and about 5mm diameter total. The inductor passes most of the current in the audio band, so the resistor normally isn't dissipating much power at all.

Fair comment on the collector resistors. The main thing that destroys them though I think, is how inductive they are. The best kind of resistors to use here are low inductance, which isn't normally the case with most cheap wirewounds.

The two resistors and capacitor that you see on the current source are a "bootstrap" to allow the current source to come up to stability quickly. I think this is to eliminate the turnon thump. You might find it is fine without these parts. Quite a lot of amps don't use them.

Another alternative current source is to use a diode based voltage reference instead of another transistor. A 3mm green LED makes a good low noise reference. Here is a good page showing a number of different types.

The turnon thump is not much of an issue. If you are putting a "DC Protect" circuit in to disconnect the speakers via relays (for if the amp goes faulty), it is a simple matter to add a 2-3 second delay on powerup here to counteract the thump.

I wouldn't call myself an expert on these matters, in fact a lot of it I have just learnt by messing about and looking at lots of schematics. I should probably get myself some books and learn the hardcore theory and maths behind it all. While it's not a sure fire way to get a working design, I have done a lot of messing around in SPICE and have learnt from there whats bad and whats good

[tcpip]

Added two base resistors R39 and R40 for the two CCS transistors, as per feedback from you and because they were there in the original schematic anyway.





I'd like to separate out the two CCS, specially if it means just adding one more transistor and resistor, but I'm still studying the board for available space.

[tcpip]

Originally posted by jaycee
Your resistor/inductor won't be that big, i think. On my own circuits it's only 12.7mm long and about 5mm diameter total.

Oh, okay, that's cool. How many turns and what gauge of wire do you use? Slone's books always talked about 18-20 turns of 14-16 AWG wire, wound on a half-inch-dia former.

Fair comment on the collector resistors. The main thing that destroys them though I think, is how inductive they are. The best kind of resistors to use here are low inductance, which isn't normally the case with most cheap wirewounds.

Absolutely. I don't know where to get those good non-inductive resistors. And I don't want to buy boutique "audiophile grade" resistors for two or three dollars each, I guess. I'm a learner... I'm not a world-beating amp designer.

The two resistors and capacitor that you see on the current source are a "bootstrap" to allow the current source to come up to stability quickly. I think this is to eliminate the turnon thump. You might find it is fine without these parts.

This I didn't know. I'm still looking through that Web page you pointed me to, on current sources and mirrors. I believe Rod Elliott uses a green-eyed CCS for his P3A. I guess one can just use two 1N4148 in series, no?

The turnon thump is not much of an issue. If you are putting a "DC Protect" circuit in to disconnect the speakers via relays (for if the amp goes faulty), it is a simple matter to add a 2-3 second delay on powerup here to counteract the thump.

I may not always put in a delay circuit... I'm toying with the idea of embedding these amps into active speakers at some date.

While it's not a sure fire way to get a working design, I have done a lot of messing around in SPICE and have learnt from there whats bad and whats good

Other than lots more reading, this SPICE part is the part I need to do. I don't have a simulator set up on my PC, and I haven't yet learned how to use those tools. I downloaded Simetrix once (it works on Linux, and I use Linux on my laptop), and simulated a phono preamp, but that's where it ended. I'll also have to go hunting for the SPICE models of all the transistors. But I think the effort will help me a lot.

[pinkmouse]

Originally posted by tcpip
I think I'll need more details.

Good morning, and Happy New Year tcpip!

Okay, I misinterpreted the topology of the output stage. I blame beer.

But look at the feedback routing, it loops all over the board to get to the stage around Q16. Not good. I would loose those oversized copper floods around the output, you don't need them, and that would allow much better routing in the central areas. Maybe swap the driver/bias sections around in that area so you can get Q15/16 much closer together. Feedback also needs to come from a point directly in the middle of the two output resistors to reduce distortion.

I'm also confused by your grounding, you seem to have lots of different grounds on the schematic, (good!), but I can only see two powergrounds on the board?