Copyright 2002-2012 Industrial Valve Audio
All Rights Reserved.
13EM7 Stereo Amplifier

Modest Components,
Elegant Sound

Here is a project that has been languishing on my workbench for waaay too long.

The circuit basics are nearly identical to the previous incarnation, which you can see here:

The only changes:

- I went with solid state rectification this time around to tighten up the B+ during heavy drive conditions.

- Still fixed bias on the output side, just like the original, but trimmer pots will now allow you to tweak the idle current to your liking.  It will come to you operating at -35 volts; right where Fred had it the first time around.

-  I am now allowing degeneration (local feedback) at the preamp stage.  I feel the small loss of gain here is more than made up for with smoother sound.

- Output Transformers on this one were sourced from a local retiree who still likes to burn his fingers on the winding machine.

- B+ Filtration is two stage CR.  The only choke I had lying around was too ugly to yoke to this amp. 

I do have to say that this amp certainly has the sound of a 2A3 - I think if you were to do a double blind listening test, most listeners (even picky audiophiles) would be hard pressed to distinguish between the two at listening levels below 2 watts output.  It has all the warmth and soundstage that you would expect from a single ended triode, but with a crisp clarity and none of the 'mush' that you sometimes get with these type circuits.  I'm becoming  more and more of a believer that running triodes at higher plate voltages and with larger load impedances than generally specified can really make for some really spectacular sound. 

No global negative feedback  is used on this amplifier.  I good friend and wise man once told me: ‘Global feedback is a cheap Band-Aid used to cover up any multitude of circuit atrocities’.  I couldn’t agree more.

All filaments are also DC fed.  The chassis is segregated into two sections with all AC confined to one side, the amplification components to the other for minimum noise.  The externally mounted dedicated bias transformer is also shielded and grounded.

Specifications and performance are listed below and also graphed.  All specs. are the real deal, measured on calibrated equipment, and not a product of my imagination or ego.

Listening to this is proof positive that modest components, properly tuned and implemented can perform way beyond what you would expect.

Input:  2X RCA stereo.  2V PP signal for full rated output.


1)    Stereo Binding posts for 6-16 ohm speakers.  Rated output of 2.2 watts/channel @ < 0.5% THD into an 8 ohm resistive load.

Frequency Response:      56 Hz - 30 KHz, +/-1 dB
                                            42 Hz - 30 KHz, +/-3 dB

SNR: 89 dB against rated output of 2.2 watts.

-    Heavy Aluminum Chassis w/ oven baked epoxy paint.
-    Grounded IEC power connection.
-    Fused input.
-    All Stainless steel mounting hardware.
-    All fastening hardware stake bonded.
-    Heavy copper ground buss.
-    Hand wired, point to point.
-    Mil-Spec Paper-in-oil (PIO)coupling capacitors.
-    Unique circuit.
-    Stabile fixed bias - no adjustments required.
-    Lifetime warranty on everything but the glass.
-    Ships completely Turn-key for operation.  Includes all NOS matched tubes and a new  IEC power cord.