Ammonite Audio
24-06-2015, 10:36
A little while back, I bought a cheap 9v battery powered MC head amp from the States:
http://i155.photobucket.com/albums/s301/hugocass/IMG-20150622-00196_zpsxlmawvz5.jpg
It cost only $100 and promised a decent performance, with this blurb:
Most audio amplifiers are high input impedance amplifiers that operate in the principle of amplifying the voltage of the input signal. Such a voltage amplifier when configured as a MC head amplifier requires a low resistance resistor (typically 100 ohms) to be placed between the input terminal and ground to obtain the required low 100 ohm input impedance. MPA-1 is a true low input impedance transconductance amplifier with a natural input impedance of 100 ohms.
It does not amplify the voltage of the input signal. Rather, it diverts the input current flowing out of the MC phono cartridge through the input circuit of 100 ohms then trans-converts it to a complimentary class A push-pull driver where the same current is driven into a loading resistor for producing the output voltage at a desired voltage gain. Since there is no voltage amplifying within the amplifier, virtually no distortion can be produced by the amplifier.
The greatest advantage of a low input impedance design is it exhibits an extremely low thermal noise from the amp's input circuitry. MPA-1 has the lowest S/N figure among many known preamplifier design.
MPA-1 operates like a step up transformer (SUT) that trans-converts the input signal level to a desired level to drive standard MM/MI RIAA phono preamplifiers. It isolates the MC phono cartridge from seeing the loading of the following RIAA preamp stage. It effectively eliminates completely the impedance mismatch issues of all SUTs.
Transconductance amplifiers usually work in open loop. MPA-1 adds a harmonic distortion nullifying circuit to reduce any remaining non-linearity and harmonic distortion to a unmeasurable level.
MPA-1 is powered by a 9 volt alkaline battery (not included) per channel. This effectively eliminates AC hum caused by all AC power supplies. The only possible source of AC hum remaining will be from the wiring of turntable, tonearm and cartridge head shell which can be effectively prevented by proper grounding and removal of ground loops.
Power consumption of the amplifier is at a micro power level of approximately 0.0018 watt per channel. A pair of 9 volt batteries typically will last from 6 months to one year.
So, how did it sound? With a Kontrpunkt B, rather good, with absolutely no noise which came as a pleasant surprise after using SUTs. Clean sounding with great detail and punch, but ultimately a little 'matter of fact' compared with the (many times more expensive Audio Note AN-S3H SUT). Nice, but it didn't invite longer term listening. So, I wondered if something clearly built to a tiny budget might be helped along with a sprinkling of better components. So, an order went in to HiFi Collective for a bunch of Takman carbon resistors to replace the generic metal films; and some Nichicon FG electrolytics to replace the unbranded ones in there. So, quite cheap stuff by audiophile standards, but the Takmans have a generally nice tone, so that's why I chose them; the FGs were the only ones that would fit. The circuit board is tiny, so it was quite fiddly to unsolder the old components and put in the new ones, but it all went well and the thing still worked, which was a relief.
Was it worth it? Very much so. After a few days of settling down, that old 'matter of factness' has been replaced with a lovely open, rich, organic sound which retains the unit's virtues of detail and punch, but in a much nicer and more musically satisfying way. It's now very good by any standard, which is not bad for a total investment of only £80 or so.
It would be interesting to hear from someone like Nick Gorham about this type of circuit, advantages, disadvantages etc. Also, how this little thing might be further propelled into truly high-end territory.
http://i155.photobucket.com/albums/s301/hugocass/IMG-20150622-00196_zpsxlmawvz5.jpg
It cost only $100 and promised a decent performance, with this blurb:
Most audio amplifiers are high input impedance amplifiers that operate in the principle of amplifying the voltage of the input signal. Such a voltage amplifier when configured as a MC head amplifier requires a low resistance resistor (typically 100 ohms) to be placed between the input terminal and ground to obtain the required low 100 ohm input impedance. MPA-1 is a true low input impedance transconductance amplifier with a natural input impedance of 100 ohms.
It does not amplify the voltage of the input signal. Rather, it diverts the input current flowing out of the MC phono cartridge through the input circuit of 100 ohms then trans-converts it to a complimentary class A push-pull driver where the same current is driven into a loading resistor for producing the output voltage at a desired voltage gain. Since there is no voltage amplifying within the amplifier, virtually no distortion can be produced by the amplifier.
The greatest advantage of a low input impedance design is it exhibits an extremely low thermal noise from the amp's input circuitry. MPA-1 has the lowest S/N figure among many known preamplifier design.
MPA-1 operates like a step up transformer (SUT) that trans-converts the input signal level to a desired level to drive standard MM/MI RIAA phono preamplifiers. It isolates the MC phono cartridge from seeing the loading of the following RIAA preamp stage. It effectively eliminates completely the impedance mismatch issues of all SUTs.
Transconductance amplifiers usually work in open loop. MPA-1 adds a harmonic distortion nullifying circuit to reduce any remaining non-linearity and harmonic distortion to a unmeasurable level.
MPA-1 is powered by a 9 volt alkaline battery (not included) per channel. This effectively eliminates AC hum caused by all AC power supplies. The only possible source of AC hum remaining will be from the wiring of turntable, tonearm and cartridge head shell which can be effectively prevented by proper grounding and removal of ground loops.
Power consumption of the amplifier is at a micro power level of approximately 0.0018 watt per channel. A pair of 9 volt batteries typically will last from 6 months to one year.
So, how did it sound? With a Kontrpunkt B, rather good, with absolutely no noise which came as a pleasant surprise after using SUTs. Clean sounding with great detail and punch, but ultimately a little 'matter of fact' compared with the (many times more expensive Audio Note AN-S3H SUT). Nice, but it didn't invite longer term listening. So, I wondered if something clearly built to a tiny budget might be helped along with a sprinkling of better components. So, an order went in to HiFi Collective for a bunch of Takman carbon resistors to replace the generic metal films; and some Nichicon FG electrolytics to replace the unbranded ones in there. So, quite cheap stuff by audiophile standards, but the Takmans have a generally nice tone, so that's why I chose them; the FGs were the only ones that would fit. The circuit board is tiny, so it was quite fiddly to unsolder the old components and put in the new ones, but it all went well and the thing still worked, which was a relief.
Was it worth it? Very much so. After a few days of settling down, that old 'matter of factness' has been replaced with a lovely open, rich, organic sound which retains the unit's virtues of detail and punch, but in a much nicer and more musically satisfying way. It's now very good by any standard, which is not bad for a total investment of only £80 or so.
It would be interesting to hear from someone like Nick Gorham about this type of circuit, advantages, disadvantages etc. Also, how this little thing might be further propelled into truly high-end territory.