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Old 19th September 2012, 12:34 PM   #1
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Default Keroes: Direct-Coupled amplifier

I recently received this article from the "Electronic Experimenter's Handbook". Written by Herbert I. Keroes of Acrosound, it details the building of a mono version of what is known as the Acro 20/20 amplifier. I'm sure this design could be bettered in many ways - notably that power supply! - so for historical purposes only. I've included the description of the schematic, not the construction notes.

So here is Mr. Kereos:
Quote:
Years ago, before the hi-fi era, an audio power amplifier was built that sounded better than any other then in existence. This unit was called the Loftin-White amplifier, after its designers, and had many ingenious features. It used direct coupling and a method of bias stabilization that was probably the first application of inverse feedback in an audio amplifier. Overall, it had a distinctly better sound - noticeably reduced distortion and better bass response.

The modern theory of feedback amplifiers provides a ready explanation for the improvement brought about by the Loftin-White circuit. Direct coupling reduced the low-frequency attenuation and phase shift, and improved the stability of the amplifier as far as low-frequency transients were concerned. Today we know that an amplifier lacking low-frequency stability sounds weak and puny compared to one that is more stable but less powerful.
...snip...

Quote:
How It Works:
Input stage V1 is used as a combination voltage amplifier and phase inverter. This tube operates with "starved" plate current to achieve maximum amplification, a condition created by the 1-megohm plate resistors (R9 and R10). To obtain best linearity and maximum driving voltage, the heater of the tube is also "starved" by means of dropping resistor R15.

Direct coupling is used between V1 and the push-pull output tubes V2 and V3. The cathode current of each output tube flows through a separate resistor, R3 and R4 respectively, which is coupled to the corresponding grid of V1.

The current feedback through R3 and R4 accomplishes several purposes. First, it stabilizes the cathode current of each output tube under quiescent operating conditions. Secondly, the cathode current is also stabilized under dynamic operating conditions to a point where the stage operates almost completely as class A, resulting in minimum distortion. Finally, the stabilized direct coupling produces an amplifier which has only one principal source of low-frequency phase shift, output transformer T1; this provides perfect low-frequency stability.

In addition to the current feedback, 20db of voltage feedback is provided by the capacitor-resistor combination C6-R13. The voltage feedback circuit is connected between the secondary of T1 and the cathode of V1a.

Response of transformer T1 drops only 1db at 5cps and 60kc. The primary halves of T1 are tightly coupled for distortionless high-frequency performance. Ultralinear taps are provided at the optimum ratio for the output tube used.
Attached Images
File Type: jpg Acro20.JPG (688.2 KB, 1649 views)

Last edited by kstagger; 19th September 2012 at 03:04 PM.
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Old 19th September 2012, 12:42 PM   #2
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and a list of the parts:

C1,C2 = .047uf/600V
C3 = 250uf/6V
C4,C4 = 40uF/150V
C6 = 100uuf / 500V ceramic
C7a/C7b/C7c = 60/40/20uf / 450V with insulated mounting
C8 = 100uF/300V with insulated mounting
D1,D2 = Silicon Diode 750mA/600V PIV (IN1096 or equivalent)
F1 = 3A slow-blow
J1 = RCA jack
L1 = 2H/200mA filter choke
R1,R2,R7,R8,R9,R10 = 1M / .5W - 1% resistor
R3,R4 = 10-ohm / .5W, 1% resistor
R5 = 2200ohm / .5W, 10% resistor
R6 = 330ohm / .5W, 10% resistor
R11, R12 = 1500-ohm, 10W, 10% resistor
R13 = 5600-ohm, .5W, 10% resistor
R14 = 220kohm, .5W, 10% resistor
R15 = 6.8ohm, 1W, 10% resistor
R16 = 22ohm, 2W, 10% resistor
T1 = Output Transformer (Acrosound TO-370)
T2 = Power Transformer 185V/200mA and 6.3VCT/4.5A
V1 = 12AX7
V2, V3 = EL84
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Old 19th September 2012, 01:01 PM   #3
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Not quite true that the OPT is the only principal source of LF phase shift. The output cathode decouplers will have a significant effect too at mid-LF, although this will eventually disappear at sufficiently low subsonic frequencies. C4/C5 combined with 1/gm and R11 etc. form a lead-lag network but because of the likely resistance ratios this could produce nearly 90 degrees of phase shift before levelling out again. A conventional output cathode decoupler has much less effect because the cathode resistor would be much smaller.

The accuracy of output bias will be similar to any other simple system which monitors cathode voltage - adequate but poorer than either normal cathode resistor bias or fixed grid voltage bias. This is because it corrects average current rather than quiescent current. The high cathode voltage, required by direct coupling, will reduce efficiency.

If the first stages anode loads (R9,R10) are high, then the phase splitter feedback resistors (R7,R8) would need to be even higher otherwise the advantage of a high anode load is thrown away. This could create a problem with Miller effect in the phase splitter, although the local feedback will compensate for this.

The OPT is clearly a high quality item, which is probably where most of the sound quality in this design comes from.

This design is a good example of how all electronic design involves compromises. It just happens to use different compromises from more conventional circuits.
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Old 19th September 2012, 02:25 PM   #4
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That circuit looks very similar or the same as one I built from an Acrosound catalogue some years ago. I seem to remember the 1M's being 510K though.

I built it as a stereo amplifier and it took a lot of time and effort to get it conditionably stable in one channel. I used Dynaco Z-565 for the outputs. I asked around on the Joenet for advice / help and the general concensus was my opt was the issue, IOW it needed the Acro OPT.
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Old 19th September 2012, 03:02 PM   #5
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I'm curious about the use of the filament starved 12AX7 - how does this increase the linearity?

I seem to remember a thread on here showing the improved curves when reducing filament voltage...????
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Old 19th September 2012, 03:39 PM   #6
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A slightly cooler cathode can reduce the electronic grid current which appears when the grid is insufficiently negative. This could be an advantage if the valve is being run with a fairly low anode voltage (as it presumably it in this circuit). The same trick is sometimes used for thermionic diodes used in radio receiver AM detectors or noise limiters, which otherwise have a slightly negative 'turn-on' voltage.
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Old 19th September 2012, 04:44 PM   #7
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Slightly reduced filament voltage in the 12AX7 also at least in some instances reduces certain types of tube generated noise. I think in this case though that the voltage drop is a little on the extreme side.

These amplifiers can sound pretty good - I had a loaner for a while.

Were I to build something similar I'd at least go with a full wave voltage doubler.
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Old 19th September 2012, 06:27 PM   #8
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I have a single Acrosound "Stereo 20" (er, it's a monoblock!) waiting for a rebuild - the tube sockets mounted to the board are pretty shot and everything else is original. It still fires up but it's definitely not something I would want to run everyday. I have a feeling it will be a long time before I see another one crop up, but eventually I will have a nice pair for a vintage setup.
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Old 19th September 2012, 09:58 PM   #9
kevinkr is offline kevinkr  United States
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There was the Acrosound 2020 which was definitely stereo, I had one on loan for a short while.

I guess there must have been a mono version as well, I've only seen the stereo amp though..

The board quality in all Acrosound amps seemed to leave a lot to be desired. The 2020 sounded very good and surprisingly the Dyna ST-35 was not that far behind - I liked both of these better than Dyna ST-70 or MKIII.
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Old 20th September 2012, 01:13 AM   #10
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It looks like there was a 20 mono amp, a 20A which could be powered off the 20(!), and the more famed 20/20.

Early stereo weirdness.
http://www.hifilit.com/hifilit/Acrosound/gold-1.jpg
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