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Old 19th January 2009, 11:24 PM   #21
Beto is offline Beto  Brazil
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scheme with modifications updated.
c1 = 220pf
R6 = 1.5K
and Q13's base. Should not this be connected at Q8's emitter.

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Old 20th January 2009, 01:23 AM   #22
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I have been simulating this circuit for a short while.

I think you may be interested in these models for the output devices that have been hand-made by andy_c. They are very accurate and will be more like real life.

http://andycpublic.50webs.com/spice_models_1.htm

Download link is at the bottom right corner.

I played with the circuit in the simulator and this happened. Distortion is lowered considerably if we give Q7 a little gain.

THD @120W, 1KHz 8ohms=.009%

THD @120W, 20KHz 8ohm=.011%

This circuit seems to show potential.

There is more I can do for this circuit but I don't know if that is the purpose of this thread. Is it your aim to improve this amplifier design?

- keantoken
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Old 20th January 2009, 07:09 AM   #23
lineup is offline lineup  Sweden
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Hey keantoken.

With +-36 Volt your nice figures are probably at 60 Watt RMS 8 Ohm.
Vrms = Vpeak/1.4142.... 60 Watt RMs in 8 Ohms = +-31 Vpeak.

It is one good amplifier. Some small goodies details.
Like R20, to reduce the open loop gain and improve linarity
and lower the feedback level a bit.

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Old 20th January 2009, 09:26 AM   #24
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Hi Lineup

The amp as shown is a rotel model 850 from 1990 or there about with improved output, like that used by bryston. Your comment about R20 is strange, Rotel changed this concept and the compensation to something different in later models to improve the sound. The resistor dissapears. There is nothing to be gained by reducing open loop gain, just more audio myths.
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Old 20th January 2009, 10:59 AM   #25
lineup is offline lineup  Sweden
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Yes, I know
We have discussed this before.
Most logic is the opinion that maximal gain = maximal feedback correction
gives best result.

But here there is another opinon, too.
That too high feedback level is not only a good thing.
Eventhough it is not easy to explain why ...
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Old 20th January 2009, 12:33 PM   #26
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Lineup I know the arguments, and have played a lot with type of thing, I dont lower the gain by resistor to ground or rail, but in the vas it self, i find no benefit or harm done with the sound, to be honest didnt change the sound at all except when driving the amp real loud, with the smaller gain the distortion became hearable. I see rotel have gone back to using it in there more recent offerings.
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Old 20th January 2009, 03:45 PM   #27
Beto is offline Beto  Brazil
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Ok keantoken very good observation made their changes planned for you. but noted that the values of R9, R11 = 250 is right? in my simulation put in each 220 ohms resistor.
Q8 modified resitores base for approximately 70ma of Bias.

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Old 21st January 2009, 05:02 AM   #28
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Hi Beto.

Your simulation probably would have looked like mine if you have used all same transistors I did. In my opinion you should get rid of the TIP4x devices, as these are made for power supplies and such. I don't think they are meant for Hi-Fi audio. I have made some more modifications and the circuit simulates quite well now. Look at the bottom of this post for THD figures and schematic.

Quote:
Originally posted by lineup
Yes, I know
We have discussed this before.
Most logic is the opinion that maximal gain = maximal feedback correction
gives best result.

But here there is another opinon, too.
That too high feedback level is not only a good thing.
Eventhough it is not easy to explain why ...
I have not tested this sonically, but here is some experience from simulations:

1: If you use, say, 2N2222 transistor for the LTP, you will get krappy HF distortion even though you have a lot of OLG. Why? Because of Cob. The 2N2222 has Cob of 22pF! This is as high as the usual miller cap, and it increases distortion at high frequencies. It is best to use Low-Cob LTP transistors.

2: Using low-Cob transistors generally in an amplifier (except sometimes for the output stage), you don't get harmonics at 20KHz that trail on to the horizon. Instead, you will have the same harmonics you get at 1KHz, but a little more.

Not Ft, Cob. Since Cob provides a HF short, it causes distortion to be distorted, causing said harmonics. The BC857B wouldn't be my favorite device to use in an LTP, because if we switch over to an NPN LTP we can use the 2N5089 which has a gain of 1.4K but a Cje of just 5pF! The important thing is a good Cje to Hfe ratio when choosing LTP devices. EDIT: Wait, unless we split the rails we'll have to use MPSA18s here, which have a worse

3: The rest of the amplifier should be linear. If the output stage is not linear, then the amplifier will be working against itself. In this case, the output stage's distortion will only be divided by the OLG. It is like dividing a number by 10. You will always get a non-zero number. You can keep dividing by 10 but the number will never be zero. If the LTP or VAS has high Cob, this distortion will also be circulated in the amplifier, providing harmonics that trail on and on to the mid-KHz.

I would have posted a version sooner if I was sure that the aim of this thread was to improve upon the design.

Here is my modification to the circuit, leaving in the output stage which I think is the most unique part of the circuit.

THD at 60W, 8ohm, 1KHz=.0012%

THD at 60W, 80hm, 20KHz=.0036%

- keantoken
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Old 21st January 2009, 02:00 PM   #29
Beto is offline Beto  Brazil
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OK keantoken you are correct in their explanations. but we will await the opnićo of Stee the original project was posted by him. what he thinks of the changes made in his project.
also think the project exceed my limitations in this area ehheehehehe still have much to learn from you great masters in the art of mounting amplifier.
thanks to all

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Old 21st January 2009, 02:32 PM   #30
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Its not always a good thing to change to NPN LTP, using PNP has its benifits, think in terms of rbb, besides what you gain with lower cob at the LTP you will lose with higher cob at the vas, theres a lot of compromises to be made. A tranny with very high gain and somewhat lowish cob is not that bad either, keep in mind that the higher gain device has high early voltage and this offsets the benefits of very low cob and lower gain.

Lineup was speaking in terms of lowering OLG by shorting the vas to ground or to a rail, the R20 in the spice schematic. A debate wich has lingered on these threads for ever. Is c6 and c7 really needed with this output arrangement ?

That cob is bad for amps is very true, even in outputstages this should be looked at, but not much you can do with standard output stage except use trannies with the lowest cob and highest early voltage.
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