A Simple 10W Direct-Coupled Class-A Power Amplifier

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mediatechnology
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A Simple 10W Direct-Coupled Class-A Power Amplifier

Post by mediatechnology » Wed Sep 10, 2014 3:24 pm

Introduction:

A few days ago I got the wild idea of trying the Dual Class-A headphone amp board to drive a pair of Minimus 7 near field monitors.
The link for the headphone amp project is here: viewtopic.php?f=7&t=509
With an 85 mA idle current and small heatsink I was pretty sure it would not be able to produce much power without distorting heavily.
I shorted the 33 Ohm build-out resistors normally used for 'phones and put some voltage gain ahead of the amp since its unity gain.
About 10V p-p (1.5W) into the 8 Ohm speakers was a good listening level; at 20V p-p (6.25W) it was as loud as I would want to monitor in the near field without fatigue.
The original acoustic suspension Minimus 7s turn out to be pretty efficient.

How did it sound?

Amazing.
Gone was the low-level grit I had grown accustomed to using 35W-Ch amps driving these efficient speakers at 5% to 18% of the amp's rated power.
It seemed brighter with more detail and better imaging even at low levels.
It made me want to box one up.

How did it measure?

It measured awful.
Really bad. At 6W about 1.6% THD.
At lower levels it measured outstanding.
All I needed was more idle current. Lots more.

So what to make of this?

I started doing some research on 10W class-A amps and found the work of JL Linsley Hood, Pass (Zen), Rod Elliot and others.
The references to those works are at the end of this post.

Elliot suggests that the optimum bias current for a Class-A is about 75%, or 0.75 X Iload-peak.
My little overstressed amp was running, at the 6W level at 8.5%, or 0.085 X Iload-peak.
10W into 8 Ohms is about 12.6V and 1.58A peak.
Iq should be in the range of 1.2A...

But it still sounded good. Hmmm....

Then a light switched on. I finally "got" class-A power amplification.
Generalizing, class-A amps have highest THD at maximum level.
Class A/B or Class B amps have their lowest THD typically just below the clipping point and rising THD with decreasing input level.
Duh. You can hear the difference.
The difference is in the detail.
Maybe the "golden ears" were right.

Taking it further:

I knew that a THD spec of 1.6% at 6W wouldn't fly.
The question then became "Could the Dual Class-A driver using the DRV134 be used to drive some really big output devices on monster heatsinks?"
The answer is yes!

Image
Dual 10W Class-A Amp Stuffing Single Channel
The emitter resistors are 0.22 Ohms It's really this simple to drive big external transistors.


Image
Dual 10W Class-A NPN Heatsink Single Channel
The 2SC5242 TO-3P device is the NPN Output. The BD139 is the Vbe multiplier and provides thermal feedback.

Image
The 2SA1962 TO-3P device is the PNP Output.
Two random NPN/PNP device pairs had Vbe matching around 1 mV.


Image
The two heatsinks are bolted back-to-back to form a stack and thermally couple the two pairs.

Image
Flying leads from the heatsink stack connect to the Dual Class-A board.

Image
Schematic of a Simple 10W Direct-Coupled Class-A Power Amplifier Using a DRV134, 2SA1962 and 2SC5242.

The idle current for both channels is about 2.5A or 150W with +/-15V supplies.
The transistors run about 85 deg C.
On the prototype for use with a regulated bench supply I wired the collector leads directly back to the circuit board and made the power supply series resistors jumpers.
When I build it for use on the bench I may use a open-frame linear supply I already have.
For unregulated power supplies, the collectors can be taken directly to the power supply and the DRV134 pre-driver supplied by Zener regulated shunts on-board or a regulated sub-supply.
(The 0.22 Ohm emitter ballast resistors can also be off-board if desired.)
The DRV134 should be operated at +/-18V maximum.
The power amp has about a 4V saturation voltage so a +/-13V peak output requires a +/-17V supply.

The circuit description for the Dual Class-A schematic shown above can be found in the headphone amp documents: viewtopic.php?f=7&t=509

How does it measure?

Image
8 mW into 8 Ohms Iq 910 mA no analyzer attenuation

Image
80 mW into 8 Ohms Iq 910 mA no analyzer attenuation

Image
660 mW into 8 Ohms Iq 910 mA

Image
8W into 8 Ohms Iq 910 mA

A couple of notes:

For starters the Dual Class-A output runs open loop and the circuit is unity gain.
Negative feedback is local and internal to the DRV134.
Secondly, the design is fully DC-coupled and does not require servos.
The measured output offset was about 6 mV.

By comparison most other Class-A designs have overall negative feedback with about 23 dB of voltage gain.
Most power amps require the inputs to be attenuated to listen at low level with lot's of gain following the attenuation.
The Hood, Pass and Elliot design's outputs are AC-coupled to the load.

I need to wire up a second monster heatsink and listen to this now that I can get the idle current up.
I also have a design in process for an active Volume Control with Variable Width.
An active control coupled with a unity gain Class-A buffer stage does not require large amounts of gain to listen at low levels.
It does not force the listener into a less-than-optimum "attenuate then amplify" strategy.

Some interesting references:

AC-Coupled Class-A Designs

"Simple Class-A Amplifier," JL Linsley Hood, Wireless World, April 1969.
http://www.keith-snook.info/wireless-wo ... 20-%20.pdf

"The Zen Amplifier," Nelson Pass.
https://www.passdiy.com/download.php?do ... zenamp.pdf

"Death of Zen (DoZ) - A New Class-A Power Amp," Rod Elliott (ESP).
http://sound.westhost.com/project36.htm

Some Clever DC-Coupled Class-A Designs

"A10"
These designs are DC-coupled. The bias scheme in the second and third figures use a symmetrical diode bias.
In some ways it's similar to the "mirror" Vbe bias scheme but rather than use a single Vbe reference and it's inverted compliment, it uses two diodes.
Both DC and AC feedback in these circuits are taken from the output.
Thus, the output is closed loop.
http://www.mimave.net/chibidac/mdd2955.html
https://ka-electronics.com

Plandemic Series: "Pland3mic - Indoctornation" https://proaudiodesignforum.com/content ... nation.mp4
Plandemic Series: "Plandemic - Doctors In Black" https://proaudiodesignforum.com/content/Plandemic.mp4

CDC: "For 6% of the deaths, COVID-19 was the only cause mentioned. For deaths with conditions or causes in addition to COVID-19, on average, there were 2.6 additional conditions or causes per death." https://www.cdc.gov/nchs/nvss/vsrr/covi ... /index.htm

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JR.
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Re: A Simple 10W Direct-Coupled Class-A Amplifier: Preview

Post by JR. » Wed Sep 10, 2014 4:46 pm

I used to drive speakers with my old Peavey headphone amp... :lol: Turns out I would melt the thermal fuse in the 1A wall wart, so I modified the PS with a resistor in series with the PS rectifier to effectively power limit the headphone amp making it hard(er) to kill with silly loads.

Class A topology in theory should be even lower distortion than class B or A/B because you no longer have to deal with power device turn-on/turn-off times. I apologize for not paying attention to you headphone amp design before, but if I read it right you are operating the output power devices open loop. This will clearly introduce a nonlinearity due to Vbe change wrt current. Increasing the class A bias can minimize this at low level.

It has been decades since I paid close attention to this but IIRC the japanese patented several dynamic class A variants back in the '70s/80s where the power devices never cut off completely, while the class A bias was varied dynamically to keep the output stage dissipation manageable. (I also recall some interesting exchanges on the internet maybe 10 years ago when that crazy russian thought he reinvented class A, thankfully I have forgotten his name now.)

I developed an automatic class A/B bias schemes back in the early '90s that didn't involve temperature compensation, but never put it into production. I had a boss at peavey who was unwilling to let me use my unproved bias scheme in a production unit (that p__ssy) :roll: I had even proved my design worked on a cheap guitar amp using plastic darlington output devices that are notoriously hard to bias using normal Vbe matching.

Have fun, might be worth investigating what was in your old 35W amp that sounded so nasty (null test?)

JR

PS: Class A amps are a winter sport... It's still hot in MS. 8-)
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier: Pre

Post by ricardo » Wed Sep 10, 2014 8:11 pm

The 10W Class A JLH design is actually the only 'non-accurate' amp that was preferred in my own Jurassic DBLTs[*] within its power limitations. It was obviously coloured compared to the 'straight wire' but liked.

It has a distortion profile very like a good valve amp which Wayne has identified. I 'think' the sound is due to the single device input stage and relatively small feedback making that the main distortion source.

I've toyed with the idea of making a big Class B version with the same nice distortion profile but probably won't get round to it this Millenium :D

Most modern Class A amps use it to get 1pp zillion THD so sound like other good transistor amps.

[*] The small number of valve amps I've put through DBLTs were in fact more accurate than the JLH. The LEAK TL50 and a VERY expensive Luxman designed by some famous (pseudo?) Western guru.

My DBLTs ask 2 questions.

The 1st "is there a difference that can be reliably detected by my true golden pinnae panel?"

Only if the answer to this is "Yes" do we ask the 2nd question which is "is there a preference?"

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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier: Pre

Post by JR. » Wed Sep 10, 2014 8:36 pm

+1... I recall when I was "helping" as an outside observer when Peavey's analog group designed a high performance (tube) mic preamp. My concern was that the design Jack Sondermeyer did was too clean. :lol: I even got him to install a jumper so I could remove some of the loop gain margin, to make it sound more "tube like". The good news is no customers complained about it being too clean so we removed the jumper extra distortion capability.

Marketing and brand mojo makes a big difference when it comes to deciding which "different" sound is perceived as better. I recall seeing an old review in stereo review of the rather expensive Mark Levinson hifi amp, and the sucker was not close to flat... but well respected.

JR
Last edited by JR. on Thu Sep 11, 2014 12:12 pm, edited 1 time in total.
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier: Pre

Post by emrr » Thu Sep 11, 2014 8:50 am

Nice work Wayne. Class A distortion....like finding the property boundary fence with your ears.....
Best,

Doug Williams
Electromagnetic Radiation Recorders

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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier: Pre

Post by mediatechnology » Thu Sep 11, 2014 4:32 pm

Here's a pic of the stereo unit.

Image
Dual Class-A 10W Heatsink Connections

The idle power for both channels combined is about 50W with an Iq of 900 mA/Ch.
I need to expand the range of the trimmer to hit about 1.1A.
The temperature rise over ambient is 30degC.
I estimate these Auguat heatsinks (which have +30degC rise at 13.5W) to be 2.22degC/W.
A similarly-sized Wakefield in the Mouser catalog is rated at 36degC/15W .

I've monitored the bias current.
When it's cold the start-up current is about 10% more than the final value.
After that it's quite stable with a slight negative Tc similar to the headphone amp.

I think once I add a dedicated power supply I'll take the transistor collector leads directly to the unregulated supply and power the driver board from +/-18V.

I don't know how it sounds yet but I think it would make JR a pretty good Crock Pot heater.
The temps are pretty stable and all you have to do to control the heat is change the Iq.
That's what's being regulated: The feedback is thermal.
https://ka-electronics.com

Plandemic Series: "Pland3mic - Indoctornation" https://proaudiodesignforum.com/content ... nation.mp4
Plandemic Series: "Plandemic - Doctors In Black" https://proaudiodesignforum.com/content/Plandemic.mp4

CDC: "For 6% of the deaths, COVID-19 was the only cause mentioned. For deaths with conditions or causes in addition to COVID-19, on average, there were 2.6 additional conditions or causes per death." https://www.cdc.gov/nchs/nvss/vsrr/covi ... /index.htm

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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier: Pre

Post by emrr » Thu Sep 11, 2014 11:28 pm

This is hardly related at all.....just a power amp observation.

I got a 60W class T amp for a spare set of studio monitors recently, and the other day I was running ARC system test bursts for room measurement/correction. The class T amp could not quite reach my normal volume knob position, it would take a couple bursts and then overload the PSU and shut down.

The other questions I raise in the above sentences:
ARC, eh, works ok, sounds pretty weird, but I figure I'm not necessarily supposed to like the way it sounds while mixing, it just has to translate.

Class T amp. Dayton Audio DTA-120. It doesn't sound right the first few minutes it's on, very thin and non-dimensional. Something has to be moving towards correct bias, temp, or both. When you find distortion/power plots on these my impression has been that they can look very different from amp to amp. Power rating tells little about lowest distortion point; it's somewhere in the middle. They say 'it's also a great headphone amp', but it can barely drive Sony MDR-7506 phones (32 ohm) to any listenable level.
Best,

Doug Williams
Electromagnetic Radiation Recorders

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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier: Pre

Post by ricardo » Fri Sep 12, 2014 12:28 am

emrr wrote:Nice work Wayne. Class A distortion....like finding the property boundary fence with your ears.....
Arthur Bailey of Radford, who designed the last of their great valve amps and also the modern Transmission Line speaker, was a water diviner. In one famous incident, he found a burst water main for a guy over the telephone :o

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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier: Pre

Post by mediatechnology » Fri Sep 12, 2014 9:09 am

Thanks everyone for posting.
I've enjoyed the comments.

THD Residual Analysis

I did some THD analysis of the residual using a conventional HP-334.
The measurement just below clipping, 0.04% THD, (1 kHz, 8R, 22V p-p) is similar to Visual Analyzer's FFT-based calculation.
Below the clipping level the THD measurement from the HP-334 isn't useful since 0.1% is the lowest scale.
The HP334's analyzer output remains very useful.

Near the clipping point with +/-15V rails

Image
Dual Class-A Power Amp, 8 Ohm Load, Just Below Clip with 15V Rails, 10Vdiv output versus THD Signature. THD is approximately 0.04%.

Note that the THD signature above is predominately third harmonic and, when driven into non-linearity or clipping, produces compressive distortion.
At lower levels the distortion products drop rapidly.

At -10dB

Image
Dual Class-A Power Amp, 8 Ohm Load, at -10dB "Just Below Clip," 1Vdiv output versus THD Signature which is also scaled.

The HP-334's "auto" function hasn't worked in years.
The residual scale was also increased to expand it.
You can see quite a bit of fundamental lurking in the above waveform since I am nulling by hand and it's very touchy.
https://ka-electronics.com

Plandemic Series: "Pland3mic - Indoctornation" https://proaudiodesignforum.com/content ... nation.mp4
Plandemic Series: "Plandemic - Doctors In Black" https://proaudiodesignforum.com/content/Plandemic.mp4

CDC: "For 6% of the deaths, COVID-19 was the only cause mentioned. For deaths with conditions or causes in addition to COVID-19, on average, there were 2.6 additional conditions or causes per death." https://www.cdc.gov/nchs/nvss/vsrr/covi ... /index.htm

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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier: Pre

Post by JR. » Fri Sep 12, 2014 9:46 am

emrr wrote:This is hardly related at all.....just a power amp observation.

I got a 60W class T amp for a spare set of studio monitors recently, and the other day I was running ARC system test bursts for room measurement/correction. The class T amp could not quite reach my normal volume knob position, it would take a couple bursts and then overload the PSU and shut down.

The other questions I raise in the above sentences:
ARC, eh, works ok, sounds pretty weird, but I figure I'm not necessarily supposed to like the way it sounds while mixing, it just has to translate.

Class T amp. Dayton Audio DTA-120. It doesn't sound right the first few minutes it's on, very thin and non-dimensional. Something has to be moving towards correct bias, temp, or both. When you find distortion/power plots on these my impression has been that they can look very different from amp to amp. Power rating tells little about lowest distortion point; it's somewhere in the middle. They say 'it's also a great headphone amp', but it can barely drive Sony MDR-7506 phones (32 ohm) to any listenable level.
I ASSume class T is based on the Tripath chipset. I was not a fan of those guys when they gave their pitch to Peavey back in the '90s. I asked them reasonable amplifier questions (like duty cycle) and they didn't understand my questions, or didn't want to answer (it appeared like they didn't understand). I worked with a guy in Hong Kong (OEM) who said he could get the Tripath sounding good with some modifications, and his demo sounded OK, but I never pursued it.

if interested in class D the amp designed by Bruno Putzey (Hypex) is very well regarded. If I was in the market to replace the guts in my circa 1970s DIY amp I might use Hypex.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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