DeathTrap400™ : : A Pretty Damn Good 300B Amp

[tomchr]

The noise level of the 300B amp? That's roughly 1 uV/rt(Hz) noise spectral density. I seem to recall that integrating up to about 300 uV RMS (80 kHz bandwidth).

I'd say the 300B amp is a bit overkill for headphone use, but each to their own, I guess.

The supply is as quiet as it gets. The B+ measures 20 uV RMS hum+noise at the output of the 21st Century Maida Regulator.

~Tom

[cotdt]

very nice, very nice...

[tomchr]

I'm likin' it...

[tomchr]

On my Switchmode Filament Supply thread, the following question came up. I figured the answer would be more useful in this thread, so I'm porting the question over.

Originally Posted by bekim

On my 300B amp project I'm collecting parts to build, will be only one tube on output stage, and 8 ohm speaker per channel.
[...], what output transformer I should consider to purchase in terms of primary resistance? I can see 2.5K, 3.5K, 4.2 and 5K primary and this confuses me.. Regardless the design, does the output transformer resistance makes a big difference and why, what will be your decision, and if you have any suggestion or brand just let me know.

The general rule of thumb is that you get lower distortion with the higher primary impedances at the expense of lower output power.

I think most people tend to gravitate towards 3.3 kOhm for slightly higher output power at slightly higher THD and 5 kOhm for slightly lower output power and slightly lower THD.

Personally, I use 5 kOhm and have tried emulating the effects of using different primary impedances by changing the load resistor. I found that I got the best performance at 400 V, 85 mA, 5 kOhm primary. I seem to recall getting about the same amount of output power at clipping regardless of primary impedance. But at the lower primary impedances, the THD at low power (typical listening levels) was quite a bit higher. Basically, my amp is optimized for 5 kOhm primary impedance, and after experimenting with different primary impedances I didn't see any reason to change this.

I have tried the Edcor CXSE25-8-5K ($80/each) and Electra-Print custom wound transformers (spec: 300B, 20 W, 100 mA, 5 kOhm, 4 & 8 ohm secondary, copper). The Edcor transformers are quite good but the Electra-Print are amazing. I wouldn't say the difference is "night and day" but I like the tone of the E-P better and they do seem to resolve more detail in the highs while providing firmer bass. But they're also 3x the cost of the Edcors. If you're on a budget, get the Edcor. At $80/each, it's very hard to beat on performance/price.

~Tom

[cotdt]

I compared Edcor output transformers and Hashimoto, and the Hashimoto immediately had better clarity and more treble sparkle. With a good 300B amp like this one, it's worth investing good money on the transformer.

[tomchr]

Originally Posted by cotdt

I compared Edcor output transformers and Hashimoto, and the Hashimoto immediately had better clarity and more treble sparkle. With a good 300B amp like this one, it's worth investing good money on the transformer.

I agree. If you're building with 300Bs, you're already aiming high. Save up some more cash. Get good OPTs. Don't put a 3-cylinder engine in a Ferrari.

~Tom

[bekim]

Tom,

Thank you for complete replies in both threads.
You answered in all questions I was looking answer from somebody who has experience on building 300B amp and circuits around it, perfect.

As per transformers you are suggesting, I search in the sites and found very good reviews and customers are satisfied with them. In my location where I live, it's a bit problematic to receive transformers from overseas and customs here is pain..
In my neighboring country, are few companies which produce output transformers, among them is trafomatic.com and they offer dual C core outputs and standard ones.
I will see how much will cost and I might order pair of them.


thanks

bekim

[bekim]

I know - too many questions


Ok, I can find mostly two types of output transformers (OT): EI and double C core. Now, more than half of people are suggesting EI core towards double C core, in terms of sound quality; warm sound, natural vocals, etc..etc..
I contact through email this guy who makes OT in poland - ogonowski.eu, and he recommends EI rather than C core..

What you say..? I know there isn't perfect part for certain job, but just your experience. I appreciate that.

thanks

bekim

[KatieandDad]

Youch !!!!

A PAIR (2 UNITS) OF HASHIMOTO SE OUTPUT TRANSFORMER H-30-3.5S

[bekim]

Yeah..

doesn't show are they EI or C core ;-)

I found this..
what are the advantages of double core output transformers vs EI ?