DIY Audio Projects Forum

MCA - Son of ZCA
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Author:  mwhouston [ 13 Jul 2012, 23:44 ]
Post subject:  MCA - Son of ZCA

Talk about put the pressure on. The ZCA has been a huge success for such a simple cct. Lately the ZCA has taken a renewed interest and foolishly I suggested a bipolar version with a 2N3055. I must confess I had thought about this for some time but have 6 other projects I would rather start. Anyhow I have come up with the bipolar prototype based on the ZCA. Not only that many pro-builders claim Fets will never sound as good as bipolars. Of course this argument has no conclusion but here is your big chance to build a bipolar version of the the ZCA though there is still a Fet in it (jFet).

Because this version has two active components I feel it should not be called a ZCA. So this amp falls under a general category called "Minimal Component Amplifier" or MCA! It worked first time and sounds OK. Here's the schematic.

Not hard to see that it shared a few areas with the ZCA. Because the hand drawn schematic is a bit rough I'll talk you through it. Firstly the B+ is about the same, 24 or 28 volts are fine. If you look at the images you will see my test PS uses a 9mh Xover choke. This works fine. The load is still 15 ohms at 20W in this case. I turn the bias up slowly until I have 15V across the load resistor which means 1A through the 2N3055. 4700uf snubbed by a 10uf poly and snubbed that with a 0.1uf poly is the same as the ZCA. I added a 1ohm 5W (one ohm) resistor in the emitter leg of the 2N3055.

Because the 2N3055 would have required a fair amount of bias current to operate I used a 2SK170 BL jFet to supply bias current for the 2N3055. These are connected as a darlington pair. This allowed me to keep the same bias cct. I used for the ZCA. I increased the bias stabilizer cap to 100uf 25V. A 1M (one meg ohm) resistor still feeds the bias current to the jFet. Also I used a 2.2uf input cap and a 200 ohm blocking resistor to feed the gate of the 2SK170. Output caps and input caps can be varied in value so experiment. I'm only dropping 15V across the load resistor but again experiment. I set the bias voltage once and it remained stead over 20 mins or so and the 2N3055 stayed pretty cool.

Author:  ziggy [ 14 Jul 2012, 01:07 ]
Post subject:  Re: MCA - Son of ZCA

Thanks for taking the time out to do this for us Mark............mucho appreciato!! :up:

So, ???? many watts does the output produce?


Author:  mwhouston [ 14 Jul 2012, 01:49 ]
Post subject:  Re: MCA - Son of ZCA

Into a resistive 8.2ohms it only produced about 1.2W RMS at 1Khz. But I have not played with any component values. The -3db points are <10hz to 125Khz!! The SQR wave at 1Khz is perfect and OK at 100Hz but at 10Khz the leading edge is well rounded off.The amp has a gain of about 4db.

Author:  mwhouston [ 14 Jul 2012, 02:54 ]
Post subject:  Re: MCA - Son of ZCA

I just tried lowering the emitter resistor. I dropped it down to 0.5ohm. The volume picks up marginally. Really I think it best left at 1ohm. Once again experiment. For those who are not sue about the darlington pair set-up: the drain of the jFet is connected to the collector, the source connected to the base and the gate to the bias network and input cap via the blocking resistor.

Author:  dtsup1 [ 14 Jul 2012, 03:30 ]
Post subject:  Re: MCA - Son of ZCA

Mark, thanks for the MCA. Am eager to try it out but have to wait till Dec. Any listening impressions? How different does it sound to the original ZCA?

Author:  mwhouston [ 14 Jul 2012, 03:37 ]
Post subject:  Re: MCA - Son of ZCA

dtsup1 wrote:
Mark, thanks for the MCA. Am eager to try it out but have to wait till Dec. Any listening impressions? How different does it sound to the original ZCA?

Only listened mono on a small speaker.

Author:  mwhouston [ 16 Jul 2012, 02:09 ]
Post subject:  Re: MCA - Son of ZCA

Here are the SQR wave results of the MCA. Only bad one is at 10Khz. The bottom signal is the output into 8.2ohms.

Author:  mwhouston [ 19 Jul 2012, 07:35 ]
Post subject:  Re: MCA - Son of ZCA

I just looked at a square wave analysis chart which shows various SQR waves and what may cause them to not look perfect. I'm cool with the 100hz and 1Khz results but would like to see some improvement in the 10khz result. According to the chart the leading edge rolling off is a result of excessive low frequency.

I have never used a 2.2uf cap (that's big for me) as an input cap. Usually they are more around the 0.5uf mark. Tomorrow I will try reducing the input cap and other minor things to see if I can get a better 10Khz SQR wave result. Then the "proof of concept" prototype will be disassembled. At this point I have no intentions of making a "production" MCA.


Author:  mwhouston [ 19 Jul 2012, 21:00 ]
Post subject:  Re: MCA - Son of ZCA

A hard day in Da Lab can bring some good results if you go in with a plan, hope and more than a whim. As posted above I felt the amp (the MCA) was swamped with too much low frequency to draw the 10Khz SQR wave cleanly. Its more about the rise time capabilities of the amp. May even be described as the "speed"of the amp or punch or slam. With a large input cap (2.2uf electro) the rise times at higher frequencies (particularly in a SQR wave) was too long. The wave form analysis chart I used helped to identify this.

The result: By dropping the input cap to 0.1uf (poly) the "speed" of the amp increased with NO detriment to any other performance specs. The -3db points are still 10hz to 125khz at 1W into 8.2ohm resistive load. The SQR wave at 1khz is still perfect and the 10khz SQR wave is a lot better. I hope to post the 10khz SQR wave with the input cap set at 0.1uf soon. This amp is now ready for prime time. We need a brave builder to build the first one. Who will it be? Why not build an amp with switchable ZCA/MCA selection. Its the same PS and biasing cct. One switch should be able to handle the change-over.

I have taken a longer listen to the amp. Classical, Opera, pop and rock. Currently listening to Christina Aguilera. The sound is punchy and driving through my small test speakers with 4" Al/Mg cones and die-cast enclosure. I may just have to build a complete amp. This may be just too good to pass-up. Impressive for a watt or so. From what I can hear I think the MCA out performs the ZCA!! You do need a preamp the gain is only about 4 or 5db though my portable CD player has no problems driving the amp to good volume from the earphone jack.

Author:  dtsup1 [ 19 Jul 2012, 22:02 ]
Post subject:  Re: MCA - Son of ZCA

mwhouston wrote:
From what I can hear I think the MCA out performs the ZCA!!

OMG :hot: :confused: :up:

I wish I could build one right now. Hopefully Kramme will get around to building it =)
How about testing them on your Osborns? :lildevil:

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