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PostPosted: 20 Oct 2010, 20:47 
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rock4016 wrote:
Since this is going to be a "high end" performing amp, have you considered mono-blocks? The PS is not done you still have time to weigh that option.

I agree. With the big dollars spent for premium 300B tubes and exotic coupling caps a mono approach seems like a logical choice and really won't add much cost and you get better objective performance (lower crosstalk).
mwhouston wrote:
Not considering mono-blocks at this point.

A dual mono build does not have to be mono-blocks per say. You can have dual power supplies but in a stereo chassis since the driver tube is shared.
Cheers

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PostPosted: 20 Oct 2010, 20:54 
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Given the nature of the project, you could go with Jensens. I just can't bring myself to recommend anything more expensive. :blush:

I won't tell you what we in the the advanced fighter avionics world have found to be the best capacitor in terms of noise, ESR, reliability, repeatability, and overall quality. You'd be shocked. :shock:

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PostPosted: 21 Oct 2010, 02:24 
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rock4016 wrote:
If you don't mind the size I'd use these for filter caps.
https://www.surpluscenter.com/item.asp? ... 5&catname=

If you are size restricted then these would do great.
http://www.partsconnexion.com/capacitor ... mtube.html

Mate, top suggestion. I have a draw full of motor starts and have been busting to find an amp I can use them on. I know we may be setting some high working voltages with this amp so I'll have to take a close a look at there V rating.

The Mundorfs 100uf 500V are perfect too. All great suggestions. Love it.

Matt: Please check the Mundies in the post above. Im looking at the 550V 100uf units. I think I only need one after the choke with a 30uf motor start prior(?). Is it OK to order these while the Auzzie dollar is good. Would you suggest bigger e.g. 200uf for after the chokes??? Let me know ASAP.

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PostPosted: 21 Oct 2010, 08:55 
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Motor starts are not like motor runs, they aren't designed for contunious use, make sure the ones you use are motor runs.


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PostPosted: 21 Oct 2010, 10:10 
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Suncalc wrote:
I seem to have recovered my health and as such have tackled the issue of setting our overall frequency response for our 6SN7/300B amplifier. However, before I begin with the details, I believe that a few comments on frequency response in general are appropriate.

The required frequency response of an amplifier is a careful trade off between the necessary bandwidth to reproduce the signal to be amplified and the signal distortion and degradation which results from an excessive bandpass characteristic. If the bandwidth is too low (or misplaced), critical content will be stripped from the signal. If the bandwidth is too large, the amplifier can suffer from bias excursions, intermodulation products, and high frequency oscillation.


That's the first mention I've heard of bias excursions due to overly large coupling caps and it's an excellent point most people overlook. This website has an article and a calculator I found useful:

http://www.pentodepress.com/home/amplif ... excursion/

I have one minor disagreement with your design philosophy. Although you can't hear 10 or 20Hz, you can feel it. That's why IMHO the low frequency response of the ears should not be used as a basis for determining the low frequency response of an audio amplifier. I can plug my ears and still "hear" (actually feel) the low frequencies through my body. That's one reason I don't care for headphones. Without real speakers there is simply no bass to feel. I would maintain that an audio system should be capable of reproducing frequencies down to 20Hz without significant attenuation to be regarded as truly hi-fi. EVERYONE can feel deep bass.

High frequency response is a different story - not everyone has the ability to hear out to 20kHz. However, I found that the 300B, being designed for audio frequencies, to be incapable of extended frequency response sufficient to cause oscillation or high frequency noise amplification problems. My amp is down about 2 or 3 dB at 20kHz without any grid stopper resistor (my hearing is only good to about 16kHz and I can't feel the treble so I don't care if the response is down by about only 1dB at my limit of hearing of 16kHz). I could be the trannys or stray capacitance creating NFB at high frequencies and not the tubes - don't know for sure - that's speculation. Or maybe my proprietary 3D wiring. :smoking:

Just my 2 cents.

After reading all the work that has gone into the design of this amp I feel like I cheated on mine by just using what was an off the shelf circuit, but one that was super easy to understand, draw load lines for, and build. I used very inexpensive PIO Russian coupling caps, Hammond OTs, simple (and cheap) isolation trannys for power, SS rectification and EH brand 300Bs and it sounds SUPER!

Can't wait to see the final design (and the final cost!). One question. Would you care to share with us the results of the testing that was done on caps that you mentioned? You've got me very curious. I've been doing a lot of listening tests on caps (and have formed some opinions about the differences in sound being related to the impedance of the source and load in the case of different coupling caps), but as an engineer I also believe in test data (when used appropriately).

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PostPosted: 21 Oct 2010, 15:49 
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An update on costs so far: $AU1500. This does include The TJ 300B/SEs, 4 X Shuguang Treasure 6SN7s, Audio Note Silver caps (X2) and a pair of EH Gold 300Bs for testing. The second pair of 6SN7s have gone into the client's preamp replacing some very nice US jobs.

I am now looking at some better filtering caps and the Mundorfs may well suite.

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PostPosted: 21 Oct 2010, 17:23 
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Suncalc wrote:
Quote:
A few months ago i found an interesting article from Gabi (Gabriel Bucataru from Sound GarageTales)
about capacitors and the noise if they are not connected right. Might be a better improvement instead of spending much $$$:
http://www.soundgaragetales.com/amp-bui ... -capacitor


Be careful what you take away from this article. He is correct in the effects for his test setup, but his conclusions are dependant on the capicator having one side at AC ground. When installed in this position, if you grip the cap and induce a hum signal in the outer layer there is a normal capicative reactance to the inner layer (i.e. the no-outer foil part of the cap). For an 0.1 µf cap this is about 26.5kΩ (1/(2*pi*60*0.1e-6)). If the outer foil end is at ac ground, then the induced signal will be greatly attenuated, if not it gets pumped directly into the amp.

However, if the capacitor is installed in a position where one end is NOT at AC ground, say as a coupling capacitor, then the effective noise path impedance is dependant on the components around the cap as well. In this configuration, any noise attenuation seen from signal induced in the outer shell is only attenuated the same as any other 60Hz part of the signal.

The only place I see where this might be an issue is in a very low signal input stage where a capacitor is being used as part of an equalization network and has one side tied to ground. In the coupling role I just can't see this making any real difference.

Just one Engineer's opinion. :)


The conclusion I reach is that for a coupling cap, if the source impedance is low (significantly lower than the impedance of the cap at a given frequency) and the load impedance is a lot higher than the cap and the source, the coupling cap may pick up more noise and send it into the next stage if the outer foil is connected to the load. If the outer foil is connected to the source, some of the induced noise will shunt through the source to ground and be attenuated. If the outer foil is connected to the load there will be a higher impedance path to shunt the noise to ground: through the cap in series with the source impedance, and that paralleled with the load impedance. The noise now has a higher impedance to ground and is not attenuated as much.

AGAIN, this assumes source and load impedances not found in typical tube amps. 26.5K is a low impedance in a tube circuit. It's more likely that the source impedance will be very high which will over shadow the impedance of the cap and render the orientation irrelevant. I guess the only reliable way to test this theory is to actually build a high gain circuit and play with the direction of the caps.

That's how this engineer sees it anyway. As always, I could be wrong. If this makes no sense I could draw the equivalent circuit to explain myself better. Interesting in any event. I never heard of this one before.

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PostPosted: 22 Oct 2010, 23:21 
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Matt: I knew request from the customer. He wants Hammond power and OPTS. I can source Hammonds in Australia. Please recommend a model for OPT and power tranni. Also recommend a choke please.

http://evatco.com.au/trans1.htm

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PostPosted: 23 Oct 2010, 11:47 
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Hmmmm....

This is a little fly in the ointment. I was just getting ready to post the final PS design. It is based on an Edcor XPWR077. But that's not what the customer wants. :bawling:

So... For the output transformers, thats easy. Go with the Hammond 1630SEA. It meets all requirements and we already considered it in our design. For the primary PS choke, I've already settled on the Hammond 193J 10H choke (or the 193M or 193Q if you can't get the 'J').

The power supply transformer is another matter. Hammond only makes two transformers that meet the PS requirements for this amp. These are the 282X or 382X ( 1000v(500-0-500)@200mA, 5v@3A C.T., 6.3v@6A C.T.). And I couldn't find either of these on the web page you linked. If I need to go to the lower voltage (400-0-400) transformer it will mean an entirely new design from the ground up.

Please let me know what you want to do.

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PostPosted: 23 Oct 2010, 15:35 
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Suncalc wrote:
Hmmmm....

This is a little fly in the ointment. I was just getting ready to post the final PS design. It is based on an Edcor XPWR077. But that's not what the customer wants. :bawling:

So... For the output transformers, thats easy. Go with the Hammond 1630SEA. It meets all requirements and we already considered it in our design. For the primary PS choke, I've already settled on the Hammond 193J 10H choke (or the 193M or 193Q if you can't get the 'J').

The power supply transformer is another matter. Hammond only makes two transformers that meet the PS requirements for this amp. These are the 282X or 382X ( 1000v(500-0-500)@200mA, 5v@3A C.T., 6.3v@6A C.T.). And I couldn't find either of these on the web page you linked. If I need to go to the lower voltage (400-0-400) transformer it will mean an entirely new design from the ground up.

Please let me know what you want to do.


Learn from my mistake. Based on my experience I agree with your choice. I was on the fence between the 1627SEA and the 1630SEA, the difference being a lower primary impedance on the 1627SEA. The 1627SEA can also handle larger DC bias currents. You'll probably get a little less distortion with the 1630SEA though, and the difference in volume will be very slight. The customer would probably be more concerned with the distorton specs.

I used the 1627SEA and I must use the 4 ohm tap with an 8 ohm speaker. Using the 8 ohm tap loads the tubes too much. The reflected impedance to the primary of my tranny is effectively twice the rating or about 5K. 300Bs seem to like 5K. They don't seem to like 2.5K. I don't know how 3.5K will sound, but it will sound better than 2.5K for certain.

Just my observations. If I could I'd swap my 1627SEAs for 1630SEAs.

Both the 1630SEA and the 1627SEA are great trannys IMHO and either would do a great job. They weigh 11 lbs each, same as the Edcor 25 watt SE models. They Hammonds are $130 and the Edcors are $90 here in the states. Don't know if that necessarily corelates with quality. The only other ovservation I have is that Hammond OTs have a lot more laminations in the core than the Edcors. That might explain the price difference. In any event based on my experience you picked a good tranny.

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