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Old 6th January 2013, 12:25 PM   #351
juma is online now juma  Germany
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Input stage looks OK, but the output is not up to the task. If you want BJTs at the output some composite stage (Super Pair, Darlington, etc.) would be more appropriate - the way you drew it, a992/c1845 can't take the heat and they will load the input stage a bit too much with the output swing you want (for 100W @ 8 Ohms you need +/-40V swing ie. 28V_RMS). Also, we know nothing about your power buffer's Zin.
If I were you, I'd rather use k2013/j313 as output pair, biased at 50mA or so.
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Old 6th January 2013, 12:45 PM   #352
jarkaa is offline jarkaa  Finland
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Hi Juma

Thanks for the info. My current buffer is Steven Dunlaps Krill outputstage. I currently drive it with e88cc anode follower. The input impedance is several hundred kohms so the load is very light.

My initial thought was to use sk2013/sj313 since I have 8 pairs of those in my drawer but using bjt:s at the output is temting for some reason.

IŽll do some research on the subject.
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Old 16th January 2013, 11:28 AM   #353
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Hi Juma!

I (maybe) have a problem with my preamp.
If I put a square-wave signal with 6Vpp on the input, my negative waveform looks like a saw on the output.
The positive wave looks absolutely perfect, no problems here.
Staying under 6Vpp --> everything is fine.

The "saw" is about 1Vpp big and isn't changing its value while going higher than 6Vpp on the input, is it a "ripple"?
Or something like "clipping"?
If I switch to sinus-waveform, everything is fine...

How much Vpp is needed on the input of this preamp to drive my F5 full?

Thank you- as always- for your answer!

Matthias
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Old 16th January 2013, 02:05 PM   #354
juma is online now juma  Germany
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GentryBa,

1. Do all the resistors have values as in the schematic from post #53 and if not, what is different (what are the exact values of R8 and R9) ?
2. What is the value of PS voltage ?
3. What's the voltage drop across R4, R5, R6, R7 ?
4. What waveforms are problematic - at preamp's input or output (look at both of them and see if it's the case of garbage-in-garbage-out) ?
5. What's your square wave source, was it properly loaded, what did you use as preamp's load when measuring?

If you made your preamp exactly as shown in the schematic from post #53 you need about 3.5V_peak-to-peak at preamp's input to drive the standard F5 to full power.

Pictures of problematic waveforms at preamp's input and output will help.
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Old 16th January 2013, 07:44 PM   #355
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Hey Juma!

Thanks for you help, I try to answer:

1. I built it as you posted it. All the resistors have the same values.
2. PS has got 16VDC, regulated by 78/7912 regulators to get 12VDC on the output.
3. Didn't measure, will describe later.
4. Here is the solution:
On the input everything seemed to be ok, but when I put the probe to the output, I could see the saw-ripple.
Then I switched off my pulse-generator (it's an ascel-diy from ebay, calibrated with a nice fluke in my work, very fine for the money!), and noticed a bad ripple on the output without any load on the input/output. (It didn't change when I put a 100K on the output to simulate the F5 a little bit)
(I have to say, my hameg HM512 is veeeery old and not the best at all...)

After some measuring I found out, that a 10”F capacitor between output and input pin on the 7912 helped. No ripple yet, nice and smooth waveform on the output!
I'm wondering why? But ok, now everything is fine, and I continued measuring.
Is it ok to have ca. 7.8Vpp on the output, when I put 3.9Vpp on the input?
I checked with different values, and everytime I got Vpp(in) x 2 = Vpp(out).
More than 7.8Vpp wasn't possible, but I think, this is more than ok to drive the F5?


best regards,
Matthias
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Old 16th January 2013, 08:35 PM   #356
juma is online now juma  Germany
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Quote:
Originally Posted by GentryBa View Post
...10”F capacitor between output and input pin on the 7912 helped.
Strange... 10uF cap should be between output and ground (also between input and ground), not between input and output. If it's really so, something's wrong with 7912 chip which is also nothing unusual considering the number of faulty chinese chips that are currently circulating in distributors' networks. to be safe, use the exact schematic from post #53 and add two 10uF caps - one parallel to C1 and the other one parallel to C4.


Quote:
Originally Posted by GentryBa View Post
...Is it ok to have ca. 7.8Vpp on the output, when I put 3.9Vpp on the input?
Yes. Your output stage is probably a bit starved so Zout is a bit higher (for that, you'll have to answer Q. nr. 3. from post #354) and that causes the gain to be a tad lower.


Quote:
Originally Posted by GentryBa View Post
... More than 7.8Vpp wasn't possible, but I think, this is more than ok to drive the F5? ...
7.8V_peak-to-peak is exactly what you need to drive the F5 to full power.
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Old 4th February 2013, 08:37 PM   #357
tessier is offline tessier  Canada
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Quote:

Click the image to open in full size.
Hi

Do this preamp from Juma could drive lower Z load than the F5 ?

What are the minimum Z load it could drive ?

And where do we connect for the output, across R8 ?

I presume that R7 are the feedback ?


Thanx

Paul

Last edited by tessier; 4th February 2013 at 09:03 PM.
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Old 4th February 2013, 11:17 PM   #358
juma is online now juma  Germany
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1. Yes, but not with great success - as it is, it has Zout of 1k5

2. I wouldn't go below 47k or so...

3. R8 represents the load

4. No. R7 puts J2's Gate to ground

That circuit can be used as SE to Balanced converter, or just as a SE preamp which has output in phase with input. Pretty basic stuff, but a lot can be built upon it...
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Old 4th February 2013, 11:50 PM   #359
tessier is offline tessier  Canada
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Quote:
Originally Posted by juma View Post
1. Yes, but not with great success - as it is, it has Zout of 1k5

2. I wouldn't go below 47k or so...

3. R8 represents the load

4. No. R7 puts J2's Gate to ground

That circuit can be used as SE to Balanced converter, or just as a SE preamp which has output in phase with input. Pretty basic stuff, but a lot can be built upon it...
Hi

This circuit was interesting because the guy wrote that it should be better sounding than the B1, but since I only have 2SK170 fets, I may made the B1 with gain that you did design.

Btw, Have you try that B1 with gain circuit, how does it sound ?

Thanx

Paul
Attached Images
File Type: gif B1 with gain.gif (16.9 KB, 437 views)

Last edited by tessier; 4th February 2013 at 11:58 PM.
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Old 5th February 2013, 10:05 PM   #360
juma is online now juma  Germany
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Quote:
Originally Posted by tessier View Post
...Btw, Have you try that B1 with gain circuit, how does it sound ? ...
I'd say great. Have a look here:
BF862 Preamp
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