DIY Audio Projects Forum

MCA - Son of ZCA
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Author:  mwhouston [ 07 Feb 2018, 22:07 ]
Post subject:  Re: MCA - Son of ZCA

Provide some images and listening notes. Did you stick 100% to the component suggestions?

Author:  ILoveHiFi [ 27 Feb 2018, 04:49 ]
Post subject:  Re: MCA - Son of ZCA

bugeyes wrote:
with an 8 ohm 10w resistor across the binding post I get 0.0mv :wine:
thanks so much for the help, onto stabilizing the bias

Wihout load seeing 14v at outpu is normal.
The proper way of fixing this is to connect a resistor 1M ohm at the output. I would recomend any 1/4w resistor or larger with 100k to 1m resisance.

Without any resistors connected to ground at the output, the output is floating and the voltage will be unkown.

I've seen this plorbem with one of my pre amps, during testing no resistors are connected at input, the input goes through capactor to amp. I was seeing a few volts I thought something was wrong.
Once I connected a 470k at intput instantly fixed the plorbem

Author:  Gerti [ 20 Jun 2019, 13:15 ]
Post subject:  Conversion from ZCA to MCA not successful

Today I tried to convert my ZCA to MCA.
All worked well, voltage adjusted at 12V, speaker hooked up and:

The hearing impression was terrible.

I switched between the the 3055 and the 2sk1058 for 5-6 times (Fixing screws, voltage adj............)
Nothing got better, harsh cymbals, harsh saxophone.
The smoothness, softness of the ZCA was lost.

I added some images of the parts u use.
Do they look counterfeit? The 3050 sideview looks mysterious...

I have no idea what is happening here.


Author:  mwhouston [ 20 Jun 2019, 18:43 ]
Post subject:  Re: MCA - Son of ZCA

Not sure what the problem is here. I only made a rough prototype and thought it sounded OK. I don’t think that even if the parts were counterfeit that it would explain the poor performance.

Did you put it on the CRO to see if the signal is clean?

Author:  Gerti [ 21 Jun 2019, 04:52 ]
Post subject:  Re: MCA - Son of ZCA

Monday at work I have access to a CRO.
Otherwise I would buy 3055 from different sources and have a try...........
Only the 2sk170 was difficult to get.
Any Ideas for a replacement?


Author:  ILoveHiFi [ 21 Jun 2019, 04:54 ]
Post subject:  Re: MCA - Son of ZCA

Fake transitors don't make any diffrence. Not enough for you to say "smoothness, softness of the ZCA was lost"

RS components only sells 100% geniune components all the time and gaurnanteed. They also offer free shipping even if you buy a single 1/4w resistor.
The k170 looks very geniune don't waste your time

Author:  mwhouston [ 03 Feb 2020, 06:22 ]
Post subject:  Re: MCA - Son of ZCA

max130 wrote:
Wow, thats making me proud :)
Firstly I have made a try on a piece of wood but got too much hum from the input.
After installing everything in a proper enclosure there is absolutely no hum (and i have very sensitive speakers).
The temperature is higher than with the ZCA: the enclosure gets approx. 50° Celsius.
The sound is much more direct, some sort of punchy. The best match to the Fostex i can imagine.
I have not heard a lot of that directly sounding systems even on HighEnd exhibitions.

I enjoy a lot!
Best regards

Do you still use your MCA? That looked a really good build.

Author:  mwhouston [ 03 Feb 2020, 06:26 ]
Post subject:  Re: MCA - Son of ZCA

LuPtBi wrote:
Thanks for your inputs regarding the choice of transistors. I decided to go for the BDY20 2U50 and I dont regret - it sounds very nice!! Highly recommendable for as an amp for daily indoor use. Depending on the input device (in my case a Focusrite Scarlett 2i2 (max. output voltage: 1.2V pk-pk @ 1kHz)) it's easily loud enough to disturb neighbours ;)

The housing just contains the amplifier and a small RC input-filter with 1 Ohm and 10mF on each side. Very handy! A mid-size fanless CPU-chiller sinks the heat of both 15Ohm resistors:


For the moment, I use my switching mode lab power supply and there is no hum at all. A CLC-supply (20mF-50mH-20mF) is close to be ready to use, too.
By driving the input at 28V, I burn almost 50W.

Playing with the input voltage, I only observe a clip in the positive half wave. Is this normal?

Because the MCA shows an amplification which is over a wide range independent on the input DC-driving voltage, I think I reduce it to say 15VDC, reset the bias and burn only a bit more than 10W all the time. Does this change the overall performance drastically (sure, I will shift the Q-point by some volts, but is this a 'serious' thing?)? My ears and my oscilloscope say it's fine as long as the input voltage is restricted by the preamp such as in my case to a relatively small value of 1.2V pk-pk. Unfortunately, I don't really have a detailled idea about the distortions and how to measure them..

In general, I'm curious how to calculate/estimate the damping factor, that will be provided by this circuit. Anyone an idea?

Thank you!

Do you have an image of the amp all closed up from the outside. Still using it?

Author:  LuPtBi [ 06 Apr 2020, 05:34 ]
Post subject:  Re: MCA - Son of ZCA

Hey Mark,

the amp is not in daily use right now but I regularly use it for sound comparisons and listening sessions.
Attached are pictures of the amp



Best wishes,

Author:  mwhouston [ 06 Apr 2020, 21:13 ]
Post subject:  Re: MCA - Son of ZCA


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