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Transissstor
Phono HeadAmp
“HEAD-EEK”
Are transisssstors more fun than transformers?
Last modified: 1-nov-06
While surfing the internet I frequently hit sites with wildest and most creative ideas, this project came to life looking for an answer on a question. I remember having a discussion with audio friends on one of the forums about phono amps and the correct terminology. From your MC cartridge to amp the signal is amplified and filtered several times and we discussed the naming conventions in this chain (in Dutch). Anyway, what would you call the MC to MM amplifier? Most people seem to make use of MC step-up transformers when building tube phono amps, and MC amplification in opamp design is standard and integrated in the phono equaliser.
The active version of the MC to MM amplifier would be called a head-amp so I learned. And when browsing the web I found some designs with active amplification of the MC signal. Until that moment I had used MC step up transformers of Lundahl (and still do) before my phono amplifiers dealt with the signal. But I was challenged by the transistor designs I found on the web. After all, I wanted to be able to make a comparison between both and these projects were so simple that it allowed me to learn more about transistor designs so what the h***.
Like I said, most of the times I find what I'm looking for on the web. In my case I found a simple head-amp capable of a gain of 20 (26dB) based on just a few cheap and easily available transistors. Most of the components I had in house already and even after I had all meterials at hand it took longer than expected to finish this project. The schematics used by me are of Marshall leach Jr, from the USA, who published several designs already in the past.
My requirements for this project were cheap and simple: Building the best sounding head-amp for MC cartridges with a gain of 15t times. Making use of good but cheap components limiting the project price to about €10.00 (about $10.00). I had to dig deep in my stock of components in order to avoid overspending.
Because my knowledge of electronics in general and that of transistors in particular is not very high, I decided to follow the design of experts in this field. Mr. Leach has descibed three versions of a head-amp on his web page and I decided to build the third version (common emitter). Below you find a copy of is original schematics.
The values for the components are as follows:
The power supply for this amp is made of a 9Volts battery, which is a relatively expensive solution. But as the signals we amplify are in the order of 0.1mV I will not worry too much about power consumption.
Leach describes a switch to turn the amp on and off. I do not like it. First of all: I'm always forgetting such things (careless me) and secondly with the power consumption of this amp being so low I expect the batteries to last forever.
After studying Leach' design for a while I had to design whether I would copy his or make mods myself. For a start, I rather had 15 times gain instead of the 20 times provided by the original design. This because my Tweety and Loekie amps already had more than average gain (and output levels).
Secondly, the transistors used by Leach are an american type which is hard to get in the Netherlands. I therefore asked Sodejuu for advise and he told me to get what European equivalents. I used 1Watt metalfilm resistors (Beyschlag) since I like their sound and I had enough of them in house.
After that I neede to take decisions about PCB, hardwiring, housing etc. As I do not have experience in making PCB's myself perforated boards are the closest I can get. As shown on the picture below, the amp is a dual mono design, with separate battery and all. The 9V power is floating.
The capacitors come from two bulk packages bought from Conrad, containing all kinds of caps in different values for just €2.00 per package or so. The connecting wires are microphone cable (Prefer or tasker) with cheap female and male RCA connectore for input resp. output. I kept the wires longer than necessary intentionally so that should I want to use a better housing with better connectors at a later stage that I won't be short on cable.
This is my list of components used in Head-Eek:
I
simulated the amp in SPICE and the results are exactly as desired: the amp has
a gain of 15 and a somewhat lower output impedance.
The copper shielded inner box containing the print is made of pieces of print board that are carefully cut to the right size. By soldering all pieces together one gets a box which is better protected against RFI.
This box, cabling and the two batteries are then put in the larger housing which is made of cheap iron.
in
order to be able to test the amp, I connected it to my function generator and
scope. Hmmm, not bad and very encouriging if things do work after you put it
together. But the proof of the pudding is in the eating so I connected it to
one of my turntables, the B&O 1700 which contains a MM cartridge with relatively
low output (I did not want to try it on my clearaudio/dynavector combo immediately).
It almost tricked me: I did not like what I heard AT ALL, brown sound and no high freqs. And after listening to it for 15 minutes I almost took it to the shredder. Time for a drink and a time-out; fortunately these breaks are often the most productive moments. This time too I found out why it was not what how I wanted it: The Head-Eek has an input impedance of 1K, and a MM cartridge expects in most cases 47kOhm input. A low input impedance results in a low-pass filter and that was what I heard.
After check and rechecking that the output of Head-Eek did not contain DC components I decided to give it a try on the Clearaudio/Dynavector/Tweety combination. Well, that was a lot better and I completely changed my mind on the Head-Eek. I recommend this little cheap amplifier to anyone needing a MC headamp, it's SUPER.
After a few days of listeing to the head-Eek I have decided to use the Lundahl step-up transformers in the Clearaudio setup. The head-eek will probably be used once I upgrade my MM (Audiotechnica AT-110) cartridge in the Thorens TD-166 to a MC cartridge.