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audiorob 5th July 2006 08:27 AM

problems building zen 5 amp

I'm a computer scientist who is lost in a EE's domain, so please forgive my ignorance.

I am trying to build my first DIY amp. I thought I would build the Zen 5 amp, as it can be built with point to point wiring. I thought I would build a test board and power it with my workbench power supply (before trying to build that big power supply). The max voltage on the workbench power supply is 30V, so I can get +/- 15V rails (at 3A). I _think_ (I don't _know_) this is enough voltage to at least get some noise from the amp (which is my goal before building the big power supply). Can someone confirm this guess?

I built the amp following Fig. 3 of Nelson's, "The Zen Variations - Part 5 The Complementary Zen" article. I'm as sure as I can be that I have built the circuit correctly. Because I'm using +/- 15V rails, I put two 47.5K resistors in parallel for R5 and for R6 (the value of R5 is ~24K, and the value of R6 is ~24K) to be able to use the pots to set the bias.

One of the first things I noticed was that by using only a Volt Meter, I really couldn't get the DC offset to vary much. I could not get the DC offset to vary across 0V. I tried a different meter, but with about the same results. I resorted to connecting 2 oscilloscope probes between the gate resistors and the gates (between R7/Q2 and the other between R8/Q8). With the scope, I could bias the transistors equally (but opposite polarity). The test bench power supply has an ammeter, so I adjusted the pots to fed about 1.5A equally though Q1 and Q2. The VM values across R9&10 and R11&12 matched Nelson's measurements (Ie: ~350mV). I don't remember the exact value, but the DC offset was very small (<.01mV). I've tested so many things that I'm getting fuzzy about my measurements, but I think I'm supplying either ~8V or ~12V to the gates.

When applying a 1V 1KHz sine wave signal to the inputs, I do see a sine wave at the gates. However, I don't really get much at the output.

I connected a speaker to the amplifier output and the test bench power supply ground. I connected a preamp output to the amp input and the test bench power supply ground. I applied a signal, but nothing. If I reconnect the signal generator to the input, and apply about a 3V sine wave, I can faintly hear a tone in the speaker. But 3V is a lot bigger signal that I thought the amp would need (I was thinking that ~1.5V would drive the amp to full output).
Does anyone know what input voltage is required to drive this amp?

Does anyone have any idea what I'm doing wrong, or what other tests I could do to debug this further?


PS - Nelson also mentions, "... nor is there any reason not to bypass the 47 uF electrolytic caps with your favorite lower value film types." I _do_not_ understand this capacitor preference stuff. I do not know what a reasonable value for a film type cap would be. I do not understand what the value of a film type cap over an electrolytic is. Also, aren't most film type caps non polarized? Wouldn't putting a non polarized cap in this circuit in place of the electrolytics be a bad thing? Could some one help me with this topic? Pointing me to a book or a link is fine.

thanks again,


Nelson Pass 5th July 2006 11:08 PM

After a discussion on the phone, we worked out that
it was the test power supply. (It's always something
that makes sense afterword)


audiorob 11th July 2006 11:17 PM

and it works
2 Attachment(s)
I'm sure that no one is surprised, but for the next person to tries this... Nelson was of course right. I took his suggestion and got a second test power supply and connected it to the first, using the connection point as ground, and the amp works! Now to build that big power supply! I'm sure this is old hat for most of you, but for me, it is really a thrill to get this thing working. I can't say enough good things about Nelson...


PS - it's the little yellow board sitting on the flat side of a heat sink.

Zen Mod 11th July 2006 11:38 PM

you have one big problem and you must solve it ASAP-your bench is not enough messy,and you'll hardly finish any decent amp this way

Nelson Pass 12th July 2006 09:30 PM

Typical computer guy - reserves the mess for his code


Zen Mod 12th July 2006 09:46 PM


Originally posted by Nelson Pass
Typical computer guy - reserves the mess for his code


yup-look-only digits are bouncing on this picture...
no smoke,no beer,no leftovers of any kind....
poor fella'


AndrewT 14th July 2006 12:51 PM

is that 8 readouts I see before me?

The eyes of a fly to take in all that info at once.

Can anyone instruct us how we can use a PC to collect all this info in just a few mS and then peruse the data at our leisure?

audiorob 25th August 2006 08:09 AM

working 27W@1KHz into 12 ohm load!
2 Attachment(s)
If anyone is interested, I have built one channel and one power supply, all per
Nelson's design (to the best of my ability). I have connected everything, and
done a little testing (which is about all I know how to do). :)

With a 1KHz triangle wave, I get 27W into 12 ohms (I don't have any 8 ohm
power resistors), with 3.8V input. I also looked at some square waves up to
20KHz. There is a little bit of corner rounding at 20Khz, but no overshoot or
ringing. This is a "test" setup, with long leads going to the transistors. Not
that things are bad or anything, but I think the 20KHz corners will get even
better when I mount things "for real" and have shorter leads.

Anyway, it sounded well enough for me to build the second channel, and
some mono boxes. More pictures soon.

audiorob 25th August 2006 08:09 AM


Originally posted by AndrewT
is that 8 readouts I see before me?

The eyes of a fly to take in all that info at once.

Can anyone instruct us how we can use a PC to collect all this info in just a few mS and then peruse the data at our leisure?

HI AndrewT,

Well, aside from the VM's and such, I use a PC/USB based O-scope. It is
made by EasySync Ltd, which is in the UK, I think. The model I use is a
DS1M12 Stingray. I think it can do at least 250KHz and 50VDC. I am a
CS guy, not a EE (, so I don't know much about scopes), but I think this is
on the slow (very slow?) side of conventional scopes. However, it is PC
based, and can capture data, as you asked...

Maybe I'll be able to post some pics of traces from the PC based scope
after I get this thing into a box. I'm not sure how big the pics of traces are,
but I know we have a posting limit of 1MB. So, no promises...

-- Robert

AndrewT 25th August 2006 10:28 AM

Hi Audiorob,
your PC scope will get you by. But it will miss out a lot of important information, either due to too infrequent sampling or due to software smoothing over the data to generate the pics to be displayed.
Buy yourself a real, non digital, scope about 20MHz to 50MHz for audio testing.

Traditionally, a digital scope needed a sampling frequency of about 10 times the maximum display frequency to achieve accuracy and resolution. I am told that modern digital scopes can do a lot better than this.

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