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Old 26th September 2012, 11:13 PM   #41
djk is offline djk
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Something is saturating and it is sticking to the rail a bit.
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Old 27th September 2012, 08:29 AM   #42
AndrewT is offline AndrewT  Scotland
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Q21 should be protected from blowing up. Add in a collector resistor. 2N5551 does not make a good VAS transistor. Aim for Cob <5pF and preferably <3pF @ 10Vce.
Q3 must be protected from blowing up if you add your output current limiter.
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Old 27th September 2012, 04:05 PM   #43
Vostro is offline Vostro  South Africa
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Thanks, Ill try another transistor (not sure which one yet, maybe another 2sc3788, to test) and add that resistor.
I added D3 to help Q3 during clip/saturating, I guess I should just have added another transistor to protect Q3 instead.

Ill report back after changes.

Just a thought, since sticking is at Positive halfcycle, is this causing
saturation of Q3,21 or on the Q12 side ?


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Old 1st October 2012, 03:21 PM   #44
Vostro is offline Vostro  South Africa
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After playing with changing Q3 with no improvement, I looked elsewhere.

I was using a KSE350 for Q12, since its all all plastic body and easy to mount to heatsink.
I replaced it with a OnSemi MJE350 and sticking was still there but with max output moving from 107Vp-p to 110Vp-p, so I left it in.

This morning I removed the RC network C2/R17, sticking was completly gone but with instability, oscillation appearing as ringing, even at low levels.

I then replaced C2 with a 22pf and instability wasnt visible and sticking was very minor,
I felt uneasy about the oscillation and replaced C2/R17 with just one 100pf.
Sticking is more than with 22pf and I hope the amp will remain stable.

Attached it screenshot of clipping with 100pf in circuit.

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Old 1st October 2012, 04:38 PM   #45
Vostro is offline Vostro  South Africa
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This board is using a tripple EF, stability seems ok,
Will it perform and be more stable if I convert it to a double EF ?

Thanks Vostro
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Old 4th November 2012, 03:55 PM   #46
Vostro is offline Vostro  South Africa
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Since my previous amp is working fine now, I want to make some changes.
Here is the next revision of the amp, still pretty standard design.

Again thanks for all the help so far,

I'd like to know about R17 that was suggested,
- I'm not sure about calculating is optimal value and power rating,
- at first glance I thought a nominal value of 470R to 2k should work,

I'd like to know what low power and med power transistors I could use for the VAS stage,
- If tried 2SC3953 and 2SC3788 but I can't source reputable pieces of these here.
- I also buy for RS but they don't seen to stock them,
- I've used MJE340 and they worked for me, but heard they not optimal.

I'm going to use a mix of through hole and surface mount parts (single sided pcb board, since I'm making them toner transfer)

I've not decided if I'll make the pcb with the VI limiter, or if I'll leave it optional on small breakout board.

Regards
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Old 4th November 2012, 04:31 PM   #47
AndrewT is offline AndrewT  Scotland
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forget mje340 for VAS duty.
You need high fT, low Cob, highish hFE and fairly linear hFE with change in current.
Same applies to Q12 since it has to react to the change coming from the VAS.
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Old 5th November 2012, 10:10 AM   #48
Vostro is offline Vostro  South Africa
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Thanks AndrewT your advice is always appreciated,

Now I just need some suggestions for The Low Power and High Power VAS transistors.
Through hole and surface mount, preferably available from RS Components.

Thanks
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Old 5th November 2012, 11:17 AM   #49
jaycee is offline jaycee  United Kingdom
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Unfortunately such devices are getting hard to find... the original purpose of high speed, high voltage, medium current and low capacitance transistors was in CRT applications.

Sanyo used to have some excellent devices, but since being bought by Onsemi, most of them seem to have been discontinued.

Fairchild have their 2SA1381/2SC3503 clones which are good, and you may be able to get them direct from Fairchild as samples.
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Old 5th November 2012, 11:23 AM   #50
AndrewT is offline AndrewT  Scotland
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Greg did a fantastic set of look up tables to allow you to select suitable devices.
Use it.
The resource should not be wasted.
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Last edited by AndrewT; 5th November 2012 at 11:26 AM.
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