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Old 28th May 2012, 10:11 AM   #81
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by AVWERK View Post
I used .47 ohm emitters on mine and from the same batch. The mv spread across all TO3,s was less than 15mv so no problems with current hogging as far as I,m concerned............
What is the tolerance of the 0r47 Re?
add that tolerance to the variation on the 15mVre and you may find that your bias currents could be anywhere, or very close. One simply doesn't know if one doesn't hand select the Re to better than 1% and preferably <0.2%
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Old 28th May 2012, 10:46 AM   #82
AndrewT is online now AndrewT  Scotland
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Kap,
the KSA100 is specified as a 100W ClassA amplifier. That 100W is delivered into an 8r0 load.

If you want/need/require a 200W ClassA amplifier for a 4r0 load then you should be looking at 200W into 4r0 ClassA amplifiers.

There is some evidence that lower emitter resistor values do result in lower output distortion. If this holds true for ClassA operation, then reducing the Re from 1r0 to 0r47 may give a small benefit. Keep in mind that with 4pairs of output devices the effective re was 0r25 and will become 0r118 for the proposed change. That is quite low for a +-52Vdc supply. Maybe Krell knew a thing or two after developing and building dozens of high power amplifiers.
Cordell has written a lot about this Re value and supply Vdc and Thermal Stability relationship.
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Old 28th May 2012, 02:00 PM   #83
spurlte is offline spurlte  United States
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Smile "The more you learn the more you must learn."

Quote:
Originally Posted by AndrewT View Post
Kap,
the KSA100 is specified as a 100W ClassA amplifier. That 100W is delivered into an 8r0 load.

If you want/need/require a 200W ClassA amplifier for a 4r0 load then you should be looking at 200W into 4r0 ClassA amplifiers.

There is some evidence that lower emitter resistor values do result in lower output distortion. If this holds true for ClassA operation, then reducing the Re from 1r0 to 0r47 may give a small benefit. Keep in mind that with 4pairs of output devices the effective re was 0r25 and will become 0r118 for the proposed change. That is quite low for a +-52Vdc supply. Maybe Krell knew a thing or two after developing and building dozens of high power amplifiers.
Cordell has written a lot about this Re value and supply Vdc and Thermal Stability relationship.
Thanks AndrewT, Stuart Easson gave me a bunch of 0R67--he has a ton of them, just send a private message to him--he said the same, but said I could experiment. I will read Cordell just to understand a new subject involved here--"Thermal Stability relationship."
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Old 28th May 2012, 02:34 PM   #84
AVWERK is offline AVWERK  United States
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From the same batch means the TO3,s and I did check the values of the emitters to see they were close enough not to lose any sleep over it

BTW Andrew, did you ever get your krell from the ksa 100 thread to finally work?

David

Last edited by AVWERK; 28th May 2012 at 02:39 PM. Reason: BTW
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Old 28th May 2012, 02:46 PM   #85
AndrewT is online now AndrewT  Scotland
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I have not returned to it.
I was awaiting comments from other builders. None arrived.
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Old 2nd June 2012, 10:44 PM   #86
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Thanks for the replies!


@AVWERK: Think that is indeed a good way to test the amplifier, when this beast finally works instead of:

Click the image to open in full size.

:-D. I will measure carefully how much mV difference there is between the devices. Luckily production processes of trannies have improved a lot compared to the 80's. Sometimes people even say matching is not necessary. If I don’t make a mistake Onsemi, which produces a lot of former Motorola products (like the MJ-trannies) have developped a new production process which makes the specs even more accurate. I thought it was something like a more accurate laser to make the dies, but I don't remember exactely what their improvement was.

Did Krell actually matched the trannies? For instance in my AudioAnalyse the MJ15003/4’s are from the same batch, but that's it. They are not specificly matched. There is about 4 mV spread in bias, emitter resistors are .47 ohm.

@Andrew: Aha, so the KSA is indeed designed fully class A into 8 ohm. My speakers are 4 ohm and have a 3.2 ohm dip (B&W DM604). Thus no full class A disco here Only kiddin, at 4 ohm the amplifier has for me more than enough class A power to make the neighbours mad haha.

Hmm, Krell used 1 ohm emitter resistors in the original. That is why I bought also 1 ohm 1% emitter resistors. I can’t measure THD, but it would be a nice experiment. I have bought 'Designing Power Amplifiers' from Mr. Cordell a few weeks ago, I will certainly read it.

By the way, made again some progress. I've found original 2SA968 and 2SC2238 Dean ! Came across a batch from a little webshop in Holland. I've bought a few pairs, so I have some replacements when I make a stupid mistake.

I have mounted them to the boards and replaced the unrelieable ISC's:

Click the image to open in full size.


Click the image to open in full size.


I also received the speaker terminals, don't mind the mess on the floor ;-)

Click the image to open in full size.

Drilled some holes in the cabinet:

Click the image to open in full size.


Resulting in:

Click the image to open in full size.
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Old 3rd June 2012, 09:17 AM   #87
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Hi

Great build, very high quality construction as well. But just one question ... does the amplifier work? is the design and PCB design sound?

regards

Nick
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Old 3rd June 2012, 11:12 AM   #88
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Hi Nick,

Thanks :-)!! I have not tested yet, there is still a lot of work to be done. Most parts are mounted to the cabinet, but I have to do a lot of wiring before I can test the amp. I think I start two weeks from now with wiring the PSU, mounting the caps to the cabinet and add little current and increasing voltage to the big caps slowly so they can slowly form their oxidelayer back (they haven't been used for at least 3 years). The PCB design is in my opinion thusfar sound, but I can't say for sure. Planning is to finish the amp somewhere in August when I have vacation :-)

Last edited by Kaplaars; 3rd June 2012 at 11:14 AM.
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Old 3rd June 2012, 05:49 PM   #89
AVWERK is offline AVWERK  United States
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At 1ohm I don,t think they matched anything, but at .15 they better

When you do bring it up, monitor for high frequency oscillation and if that happens shut down immediately!!
Put a trap across the speaker outputs to make it work (.1uf and 5ohm Zobel)
Doing a Bode plot and staggering poles is another whole issue on bandwidth and stability that a simple trap will fix for now.

Regards
David
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Old 3rd June 2012, 08:04 PM   #90
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That is exactely what I wanted to ask next David :-)! It takes a few weeks before I can finally fire her up, but I better be prepaired. I am curious how to test the amplifier incrementally. Is there a sort of checklist? Ofcourse I will check bias, voltages across the caps, DC offset and so on, but what is sane to check further?

I've made a large dummyload with big 2 ohm resistors. I've mounted 4 of them in series on a tunnel cooler per channel, so in total there are 8 resistors. It can dissipate at least 200W @8ohm per channel. I added one 1uF capacitor per channel parallel to the resistors to approach the capacitative character of a loudspeaker a bit more. Is addition of this capacitor sane? An oscilloscope (with two channels, so I can compare both channels) and two AC RMS mV meters (one per channel) are attached to the dummyload so I can measure accurately how much output and what kind of sinewave comes out of this 'beast' when I inject a signal. I use my computer as tone generator by the way. I have also a variac so I can make the voltage incrementally higher.

Will I see a bad sine wave (when I inject 1000Hz or so) at the oscilloscope when there are HF oscillations? And do I have to measure oscillation across the speaker terminals or at the driver boards? Do I need specialised equipment to make a Bode diagram? Or is it sufficient to do a tone sweep while monitoring dips in output power?

I want to make really sure that everyting is safe and sound when I are going to use the amplifier, so the more test, the better :-) If there are more suggestions of testing the amplifier, please post them, I am really curious :-)

Last edited by Kaplaars; 3rd June 2012 at 08:07 PM.
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