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Old 1st December 2010, 07:38 PM   #21
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Thank you, Nick, for the schematic and details.
Have you had enough listening time to describe the sound?
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Old 1st December 2010, 09:15 PM   #22
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Originally Posted by THD+N View Post
Here is the schematic. A break down of the parts I used is as follows:
Thanks for your experience!

Few questions about the results:

1. Why you chose 2.5K load and not 3-3.5K?
What is the resistance of OT primary winding (copper loss)?

2. How much hum level are measured?

3. What about frequency response?

And yes, please share your subjective impressions about amps sound!
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Old 1st December 2010, 09:21 PM   #23
THD+N is offline THD+N  United States
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Well, it seems to have good separation and imaging. My speakers are elevated with the tweeters (MTM configuration) at about ear level when I am sitting down.

When listening, the speakers seem to disappear and you can't tell that the sound stage is coming from the speakers. Instruments are well placed and balanced. I am not using any tones controls yet to try and get a sense of the sound quality.

I do not have a super critical ear, but I believe I have a good ear. There is no perceptible noise from the tweeter even with your ear right up to the tweeter. Due to the use of AC on the 2A3 filaments, there is a very slight hum coming out of the dual 6 1/2" mid-woofers, but again, you have to place your ear right up to the speakers to hear it.

Since my speakers had a nasty impedance rise about 2kHz, I made a series notch filter to smooth out the impedance and now the nominal impedance across the entire audible spectrum is 3.8~6 Ohms, so now they are tube friendly so to speak.
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Old 1st December 2010, 09:35 PM   #24
THD+N is offline THD+N  United States
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I ran some AP tests the other night and will post the results.

In the meantime, the frequency response is flat from 7Hz~22.5kHz (-3dB points)

Signal to Noise @ 1kHz - 93dB (A-weighted)

Channel Separation @ 1kHz - 65dB

No particular reason for using 2.5k for the 2A3 load impedance. I wanted to try some of the Edcor SE transformers and this is what I selected as an experiment. Of course anything from 2.5k~5k can be easily used for the 2A3 with good results.

I have not measured the OPT primary or secondary resistances yet. Will have to do just so I know.

Last edited by THD+N; 1st December 2010 at 09:37 PM. Reason: Spelling
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Old 2nd December 2010, 01:33 AM   #25
THD+N is offline THD+N  United States
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Here are a few AP graphs.

The Signal to Noise and Channel Separation are lower than previously quoted... I had quoted test figures from another 2A3 amp I was measuring last night.

Even though the SNR and Channel Separation are not great, the amp still sounds good. The Chinese 2A3B tubes I have are still somewhat new. Hopefully, in time, the distortion will improve at low levels.

My best Chinese 2A3 tubes are on loan to a friend right now. They measure about 0.3% THD+N at 1 Watt.

I apologize for the "hard to see" graph colors. When converted from .bmp to .jpg there is a loss of resolution.
Attached Images
File Type: jpg Frequency Response @ 1 Watt.jpg (66.4 KB, 227 views)
File Type: jpg THD+N vs. Frequency @ 1 Watt.jpg (82.2 KB, 206 views)
File Type: jpg Signal to Noise Ratio @ 1 Watt.jpg (54.2 KB, 204 views)
File Type: jpg Channel Separation @ 1 Watt_Left.jpg (60.3 KB, 203 views)
File Type: jpg Channel Separation @ 1 Watt_Right.jpg (59.9 KB, 203 views)
File Type: jpg Power Response @ 4.4 Watts.jpg (68.0 KB, 33 views)
File Type: jpg Signal to Noise Ratio @ Rated Power.jpg (52.9 KB, 39 views)

Last edited by THD+N; 2nd December 2010 at 01:36 AM. Reason: Spelling
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Old 2nd December 2010, 09:26 AM   #26
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Quote:
Originally Posted by THD+N View Post
Here are a few AP graphs.
My best Chinese 2A3 tubes are on loan to a friend right now. They measure about 0.3% THD+N at 1 Watt.
hi, THD+N

Super results!

Was frequency response measured with dummy or with active load (speaker)?
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Old 2nd December 2010, 10:50 AM   #27
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Quote:
Originally Posted by THD+N View Post
Due to the use of AC on the 2A3 filaments, there is a very slight hum coming out of the dual 6 1/2" mid-woofers, but again, you have to place your ear right up to the speakers to hear it.
hey, I have had tube amps in the past, but Im new to tube building

even if very little, is small amounts of hum something you just have to accept

when looking at layout, is it optimal to have signal wire running on both sides ?
wouldnt it be better to have them close together, on one side of amp ?
or maybe it doesnt matter ?
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File Type: jpg 2A3 Kryptonite (40).JPG (665.1 KB, 92 views)
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Old 2nd December 2010, 03:00 PM   #28
djn is offline djn  United States
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Originally Posted by Matt.B.H. View Post
Nice

How did you get the crackle looking paint?

Cheers Matt.
It is called "crinkle" paint here in the U.S. and can be found anywhere where spray paints are sold.
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Old 2nd January 2011, 04:22 AM   #29
THD+N is offline THD+N  United States
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Originally Posted by levelfive View Post
hi, THD+N

Super results!

Was frequency response measured with dummy or with active load (speaker)?
I always measure frequency response with Dale 4-Ohm or 8-Ohm non-inductive 250 Watt power resistors.
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Old 2nd January 2011, 04:29 AM   #30
THD+N is offline THD+N  United States
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Quote:
Originally Posted by tinitus View Post
even if very little, is small amounts of hum something you just have to accept

when looking at layout, is it optimal to have signal wire running on both sides ?
wouldnt it be better to have them close together, on one side of amp ?
or maybe it doesnt matter ?
When using AC on 2A3 filaments, you have to accept a small amount of hum.

The input signal wiring can be run down the same side or separate as in my picture. If run side by side, use a good shielded cable to minimize channel interaction.
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