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Old 30th November 2006, 10:33 PM   #71
Johan Potgieter is offline Johan Potgieter  South Africa
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Dan,

To follow - still trying to help: To get one thing right at a time (and if you cannot check at the output for absence of oscillation), it might help to decrease the feedback temporarily by making the 3K3 resistor (R8 if I recall correctly) larger - say about 15K - 22K, and remove the 47pF (temporarily) until the d.c. matters are in order. But - sorry to repeat - you do need to see whether there is instability (oscillation). The presence of that can mess up all d.c. measurements. It is simply a no-go to keep blowing things up. Can you get hold of an oscilloscope?

Regards.
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Old 1st December 2006, 07:56 AM   #72
DanDini is offline DanDini  United Kingdom
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Johan,

There seems to be some confusion here - I was talking about the mv across R19 - which is set as recommended (10-15mv) not across R7 - which I admit I haven't measured.

I will (again) double check all the values, but I have done this already - relying on a meter rather than my ability to read the bands correctly.

I can make the temporary changes you suggest - is it ok to run it up like that or is there a risk - ie if it improves matters, why not leave it with the modifications?

If you need to check the schematic it's page 19 of this pdf:

k8060 manual

I can bring the amp to work and connect it to a scope - what, where and how would you check as we've already done some basic comparisons of input and output waves (from a signal generator) but admittedly should have set it up so that it would 'go crazy' to see what was happening.

Thanks again!

Dan
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Old 1st December 2006, 08:51 AM   #73
richie00boy is offline richie00boy  United Kingdom
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Can you try swapping out the bias adjusting pot for another one? Have you used a carbon single turn type or a nice cermet multi turn one?
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Old 1st December 2006, 11:04 AM   #74
DanDini is offline DanDini  United Kingdom
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I've used the one provided - a little plastic single turn effort - 1k.

Exactly as in this picture (only black!)

picture of k8060

I can try something different of course. Should it still be 1k (as provided in the kit parts) as I notice in the circuit that's it down as 500R?

Dan
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Old 1st December 2006, 04:26 PM   #75
richie00boy is offline richie00boy  United Kingdom
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It's not a massive deal, but ideally if a 1k pot has been used that would mean the other resistors each side need to be increased (about doubled) as well. It wouldn't hurt to try a 500 ohm pot if the other resistors are as per schematic, but I'd be a little surprised if it was the cause of the problems.
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Old 1st December 2006, 07:21 PM   #76
Johan Potgieter is offline Johan Potgieter  South Africa
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Hi Dan,

Oh boy!

What am I doing in the middle of the night - and not for the first time, I saw when rereading all the posts. My apologies.

Dan, I am going to model and analise this circuit on Spice - owe you that much (that is p. 19, k6080 manual). It is a little unsatisfactory suggesting one thing after another and not seeing the effect of every change, with the danger of destroying yet more components. Such a simulation will not include casual capacitances, but will give an idea of where any sensitivities lie, if any. There are quite a number of capacitors that can influence stability, should that be a factor (i.e. C1, C2, C3, C4, C5, C6-R10, etc.)

I hope to be able to get back to you tomorrow night (it is now 22:20 here), if someone else does not come up with wisdom in the meantime. Let us hope that something will come up.

Regards.
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Old 3rd December 2006, 07:27 AM   #77
DanDini is offline DanDini  United Kingdom
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AMAZING:

I tried a 500k pot, but it wouldn't give me the values I nees to get the bias right, so I went back to a new 1k, 10 turn type.

Miracle, it now works! I've been able to use the 30-0-30 supply too! The little platic trim pot must have had a problem, although it measures ok? I cannot believe after all this hassle and all the re-stuffing etc. that the pot was the cause, but it is now working absolutely fine - or at least as well as a simple kit like this can.

Sooo many thanks to all for your input, but especially to Richie for hitting the nail on the head. It makes sense in the end as the pot was the only bit not changed or replaced from original.

Cheers,

Dan
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Old 3rd December 2006, 09:05 PM   #78
Johan Potgieter is offline Johan Potgieter  South Africa
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Dan!!

OK - I went out before reading of your success, so I suppose to relate the results of my promised analysis does not make much sense now. (I was lead astray by your finding about 15mV across R19, which indicated that your bias per RV1 was set somewhere near correct.)

Anyway, I did a few hours of analysis on the circuit including some worst case, which I do not suppose will do much harm relating it here - it may be of value to others. There are certain things, I fear, that are not kosher, even though it will "play". This has mostly to do with stability under different load conditions, especially with some capacitance, and improved distortion performance, although the latter is quite low. Also apology if this is going to be a little long.

To begin with, stability is to be reckoned with because of the inordinately high NFB factor of 52dB (400x). As a result compensation sets in after 3 KHz, which leaves quite a reactive interaction at higher frequencies; something I try to avoid.

The most serious matter is perhaps the very variable input impedance with frequency. We observed correctly that R3 acts in an amplified way, being connected to the NFB return point, but such an arrangement has a penalty - it is NFB-dependant. In this case the mid-frequency impedance is about 46K, but going right down to <3K round about 20Hz as a result of NFB phase change because of the values of C9 and particularly C11. This would manifest as an audible lack of deep bass as soon as one feeds the input via a volume control introducing some series kilo-ohms. (In the analysis my generator internal impedance was 2K.)

The best way to improve this is to increase the impedance of the whole network. A substantial increase in C11 only (to a lesser extent C9) would do the trick, but leaves one with several thousand uF; not very practical. (Also keep in mind that R3 and R8 must be equal to ensure any sort of balance in the currents of T1, T2, something that will affect the zero-currents in T7, T8 - they are quite sensitive to this.)

I ended up with R3=R8=33K, C11=25uF - 33uF (not more), R2=1K, C3=6,8pF, and C9=470nF (an electrolytic here is not the best). For some relief from early phase shift caused by C1, that value could become C1=12 - 15pF. The input changes will give an input impedance of 47K, rolling off to no less than 38K at 15 Hz.

It was mentioned that some equality in the collector currents of T1, T2 be obtained. (See the power amplifier design discussion by Douglas Self.) I fear that in this circuit Ic(T2) was about 70% of Ic(T1). I changed R9 to 4,7K to get within 4%. [The problem with a non-adjustable balanced input circuit like this is that T1 and T2 must be equal. Either you select them (which the manufacturers may have done) or you arrange for adjustment. Simple circuits can be over-simplified.)]

Finally, I wondered about the fairly large C4, C5 shunting a high-impedance circuit. Under all circumstances they adversely affected pulse response; I removed them. Perhaps someone else can enlighten me about what their purpose might have been. Overshoot also cleared up when I removed R4 (make a short), and connected about 10nF over C, E of T6.

This may sound like a redesign of something intended to be simple; it in fact simplifies it even more. I could really not find justification for the original values, both as far as stability and pulse response (related) are concerned. The intention was not redesign, simply a cleaning-up. Many other refinements are possible, as most will realise. The amplifier still does not like capacitive loads - for that the usual simple inductance in series with the load is required. But it is now stable up to no load - at least in analysis. (As stated before casual constructional capacitances cannot be taken into account.)

Also, though the Zobel network (C6-R10) showed some advantage on analysis, I have found in the past that not all circuits like that in practice. It has to do with the layout, practical loudspeaker impedance, etc.

So, this naturally puts you under no obligation, Dan; the analysis was good exercise. I am happy for your success after such frustration. You would have learnt that it is often the simplest things that bogs one down. I have had many such experiences.

Good luck!
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Old 4th December 2006, 11:30 AM   #79
richie00boy is offline richie00boy  United Kingdom
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Glad I was able to help Dan.
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