Tube Power Amp Reviews

Doshi Audio Evolution Monoblock power amplifier Specifications

Sidebar 1: Specifications

Description: Class-AB monophonic tubed power amplifier. Tube complement (each amplifier): (1) 12SJ7GT (NOS Hytron); (2) 12SN7GT (NOS GE); 4 KT150 (Tung-Sol). Maximum power: 160Wpc into 4 ohms (19dBW). Frequency response: 10Hz–80kHz, –3dB. Input impedance: 20k ohms balanced, 47k ohms single-ended. Output impedance: 0.5 ohms. S/N ratio: >80dB below 1V. Power consumption: 500VA max.
Dimensions: 17" (432mm) W × 12" (305mm) H × 21" (533mm) D. Weight: 75lb (34kg).
Chassis finishes: Natural or black-anodized aluminum.
Serial numbers of units reviewed: EVO2027, EVO2028.
Price: $41,995/pair. Approximate number of dealers: 6. Warranty: 3 years parts and labor, 6 months tubes. Manufactured in the US.
Manufacturer: Doshi Audio LLC, Charlottesville, VA, USA 22901.Tel: (917) 952-2758. Web: doshiaudio.com.

COMPANY INFO
Doshi Audio LLC
Charlottesville, VA
USA 22901
(917) 952-2758
doshiaudio.com
ARTICLE CONTENTS

COMMENTS
JRT's picture
Submitted by JRT on April 16, 2021 - 7:40pm

JA1, I always enjoy reading your measurements page(s), and I have one suggestion for measurements of any amplifier using an output transformer. You are probably well aware that at some combination of lower frequency and higher current the transformer core will begin to saturate and 3rd order harmonic distortion will significantly rise. My suggestion would be to include measurement(s) that better explore the high pass corner of the full power bandwith. That is intended as suggestion, and not any negative criticism of the measurements, explanations and other commentary you are already providing, all worthwhile. Thanks for what you do.

Jack L's picture
Submitted by Jack L on April 19, 2021 - 2:24pm

Hi

Besides nonlinear hysteresis transfer of the transformer core, high frequency distortion of any output transformer is also caused by its leakage inductance & winding capacitances, which in turn, change the loudspeaker loading impedance & phase angle.

More complicated will be: such leakage inductances of each half of the primary winding of a push-pull output transformer, like this $21,000 monoblock reviewed above, should be as little as possible. Otherwise, this will cause incomplete cancellation of 2nd harmonics, causing undue generation of higher order harmonics.

Such high harmonic distortion may be fed into the secondary winding & then into the loading loudspeaker by capacitive coupling unless there is an effective static shield between the primary & 2nd windings.

Ideally, lower plate resistance of the driving tubes, less will be such distortion at high frequencies. That's why triodes are always preferred to pentode/tetrodes which all get much much high plate resistance than a triode.

That's one of the reasons I always go for single-ended Class A power topology using triodes only - more output transformers friendly + better sound !

Listening is believing

Jack L

tonykaz's picture
Submitted by tonykaz on April 17, 2021 - 8:52pm

Nice literature here, hell of a review, applause +++!

I look to have one of these designs, one day.

Nice Work,

Tony in Venice Florida

a.wayne's picture
Submitted by a.wayne on April 18, 2021 - 1:10pm

This Doshi does measure well for a modern tooby amp ,, interestingly prices are clearly for limited sales numbers , 40K plus is a bit much for an unknown Brand, especially when considering its 3times that of the last VAC integrated tested ..

Regards

Jack L's picture
Submitted by Jack L on April 19, 2021 - 9:41am

Hi

Agreed. In fact, some $200/output watt per JA's test report for 8ohm loudspeaker load is very expensive. 8-ohm load s the standard specified load impedance of any loudspeakers, right ?

The bias adjustment of the power tubes: It looks so similar to Dynaco ST-70. I did the same bias adjustment upgrade to my ST-70 by reducing the cathode to signal ground resistor (used to monitor the "bias voltage") from original 15.6R to 2.3R. Such reduction allows me to set the optimum voltage from original 1.56V to 107mV.
Yes, it pretty sensitive to adjust as JA pointed out in his test report. But I found it so helpful to improve the sound quality.

Why? The best sounding bias topology, IMO, is ground cathode. So by reducing the "bias monitoring" cathode resistor to practically MINIMUM would help to improve the sound.

$21,000 for 106Wrms/8-ohm is pretty expensive, my friend.

Jack L

Ortofan's picture
Submitted by Ortofan on April 19, 2021 - 3:21am

... (and sound quality) of this Doshi amp compare to those of the $11K McIntosh MC1502?
https://www.mcintoshlabs.com/products/amplifiers/MC1502

The MC1502 is the regular production replacement for the limited edition MC2152 - minus the fancier chassis with carbon fiber panels - but with identical specifications and sound quality.
Hi-Fi World tested the MC2152 and found that it could deliver 220W at the onset of clipping and 240W at 1% distortion.
https://www.mcintoshlabs.com/-/media/Files/mcintoshlabs/ReviewAwardDocuments/MC2152-and-C70-Hi-Fi-World-July-2019.ashx

The Part-time Audiophile reviewed the MC1502 and concluded that it provided "true high-end audiophile sound in all its refined glory, without any rare tube types, finicky setup, or usage idiosyncrasies."
https://parttimeaudiophile.com/2021/01/24/mcintosh-labs-mc1502-power-amplifier-review/

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