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|13th June 2012, 05:45 PM||#12|
I powered up Circuit 1 on 30V-0-30V bench supply and so far, its ok.
I played some music on it and it played.
Im aware that this amp isnt high quality but its been fun and would like to take some measurements.
But how do I measure output noise,
I got access to multimeters and scope, no fft function.
I tossed a AC Voltmeter and got 1.4mVrms , 10min later when I looked again it was 0.9mVrms , this is with input pot set to min.
Crappy pot tho, doesnt seem to goto 0R.
Ill connect it to bridge and 30V-0-30V AC transformer.
|13th June 2012, 05:59 PM||#13|
In the spirit of fun and diy,
I continue to play with this circuit.
Some changes have been made.
Is 60mWq ok for 2n5401's
lol ignore random component numbers, product of changing my mind.
Q1,2 Running at ~ 60mWq (Ic ~= 1mA)
Q11 Running at ~ 60mWq (Ic ~= 2mA) (Vce = (60 -(2mA x 15000)))
Q13,14 Running at ~ mWq neglectable
Q3,12 Running at ~600mWq (10mA x 60)
Q15,16 Running at ~330mWq (Ie ~= (0.6 x 2) / 220) = (5.4mA)
Q4,5 Running at ~500mWq (Ie ~= 6.5mA + (0.5mA x 4) per Base) = (8.5mA)
Q6,7 ````\ Running at ~ 2.4Wq (Ic ~= 40mA per Transistor)
Pls advice is Currents at Transistors are ok for opperation,
and if power diss, is ok. (0.6Wq at Vas transistor worries me abit)
I struggle with PCB layouts, but I keep trying.
This needs revision, but Ill share anyway, maybe theres some comments,
Like moving tracks, components,
I'm hoping that its heading in the right direction, for a beginner.
This project has been so much fun, and ofc I have some excitement
and hope that the result is reasonable, but even if it isnt,
I still enjoy it.
|13th June 2012, 06:29 PM||#14|
Join Date: Apr 2004
Location: Norwich, UK
The only way to learn how to do PCB layout, is to do PCB layout Trial and error. You are doing well so far.
Your power dissipation sounds OK to me. As long as your VAS transistors have a little heatsink, as they do in your layout, they will be fine. A good rule of thumb is can you hold your finger on the transistor when its operating? If you can't, it's too hot.
|19th June 2012, 03:09 PM||#15|
Circuit 1 progress,
I finally got some time to put circuit 1 on a scope.
Supply was 30V-0-30V Unregulated with 6800uF smoothing.
Input was 1kHz sine wave from signal generator.
Output was 8R 100Watt Resistor in sml heatsink box.
Scope on x10 Probe.
Vas used was 2SC3953E ft=400Hmz, Vce(max)=120V, Cob= 2.1pf, hfe=100-200
At first used 6800uF smoothing caps on supply.
I got ringing as posted in attachment with output higher than ~30Vpk.
I then added my loose unreg supply board, with extra ~20mF smoothing.
The ringing was still there, so I guess its not due to supply ripple.
After staring at circuit for too long I decided to change input transistors and Vas transistor.
Ringing was still there with other input transistors,
so I change 2SC3953 Vas to a MJE340.
With MJE340 Vas ringing was gone.
Output could reach 77Vp-p (93W into Resistive load) before clipping.
Gain is even from 20hz to past 20kHz.
Last edited by Vostro; 19th June 2012 at 03:30 PM.
|19th June 2012, 03:24 PM||#16|
Im changing the values of:
Q3 will be getting 22R Emitter resistor.
Q3 was changed to MJE340.
R7 was changed from wirewound to carbon.
R8 was changed to 150R 1W carbon.
I want to change cf2, not polarised electrolytic, Any ideas?
Must D4 be grounded to supply grnd or signal grnd, I didnt see a reason to connect to signal grnd,
but still wonder.
Last edited by Vostro; 19th June 2012 at 03:27 PM.
|19th June 2012, 03:44 PM||#17|
This is Output on scope just before and after clipping.
x10 Probe used.
Output Volatge on scope display is in Vp-p.
|19th June 2012, 03:56 PM||#18|
Join Date: Jul 2004
Location: Scottish Borders
Ringing goes when changing from 2.1pF for Q3 to unspecified Cob for Q3.
I'll bet the increased Cob effectively added Cdom and increased the stability margins by reducing the open loop gain at high frequencies.
Instead go back to your low Cob for Q3 and increase Cdom across BC junction. Try 47pF to 100pF.
regards Andrew T.
|20th June 2012, 04:00 PM||#20|
I replaced C3953,
I then replaced Cdom with 47nf then 68nf then 100nf,
the top of the waveform rounded more but still rang at closer to max output.
Maybe im missing something or maybe I have bad copies of the transistors.
I put back the MJE340 and 22pf silver mica and no ringing at all, even to past clipping.
For now I think Ill be satisfied as it is.
When I buy in other low cob transistors, Ill revisit this.
Also changed C9 to 100uf, Cf2 to 470uF
I reset Bias and dc offset to 75mA measured with ammeters instead of fuses.
Before changes, the amp made a corcerning thump at switch on, and switch off.
After changes and settings noted here and previous posts, the thump is very low.
Im happy ,
Next is to build circuit two, which is evolution of circuit 1, but still very conventional.
Ill see if all the improvements in circuit 2 makes a big enough improvement to be noticeable.
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