Mpp

[Joachim Gerhard]

I understand. i will send more measurements soon. thank you for alerting me on that important matter.

[Joachim Gerhard]

I measured overload in the MPP again, this time compensating for the RIAA curve.
I could not get a consistent reading at 20Hz due to the sampling rate i am using so i did the bass overload at 100 Hz. I do not think that distortion at 20Hz whould be any higher.
I also tested the overload on the scope and got 17V P-P "without visible waveform distortion" as my old professor whould say. this is rather more then my calculatet 14.1 V. the reason is that i defined overload as 0.1 % distortion. the improved version of the MPP will swing to 31 V P-P when i drive the output stage on +-18V supplys ( the maximum alowed with the OP Amp choosen - 5V because it can not drive rail to rail). i rather whould not recommend that because i found that many OP Amps sound better on a lower supply like +-12V to +-15V. Thermal problems may come into play here. Try it out, i whould not like to start a discussion abbout that here.

[reinhard]

Schönes Hemd

[Joachim Gerhard]

I have build an unbalanced version of the MPP in the meantime. I used 2SB737 and 2SD786 selected for Hfe and VBE this time. As expected distortion is much lower and gain is higher due to the much better Hfe matching. After this experience i can not recomment the MAT02/03 combination any more. The Hfe matching is poor because the PNP type has much lower gain. Instead of the hard to source ROHM transistors other types could be used, for example the ones that Syn08 recommends. You could also make your own cheap low noise transistors from a row of 4 BC550C and BC560C or even better from 4 2N4401 and 2N4403. The unbalanced version has a noise advantage of 3dB compared to the balanced MPP. i have reduced the circuit to the absolut minimum so it is a good project for DIY. i build only the head amp because i have a lot of MM stages around. i needed around 6 hours to make it with the help of one employee who made the mechanical things so it could be done on one weekend. i will give the full diagramm with all values later. Now i go home for a listen.
P.S.: i did some other changes for improved performance. first i thermocoupled the inputstage (see picture). bias is now quite stable and with some trimmimg of the constant current source DC coupling is posible. i used a good quality coupling cap here. i changed the constant current source for the foldet cascode. on the first prototype i used a half LSK 389 near IDSS, not a very high impedance source. i changed to a BF246A set to 9mA. i found that stable bias is very importand in open loop circuits and having compared the fet to a simple resistor i watched a major improvement in distortion when the fet was in action. this 9mA rases the idle current in the foldet cascode stage and it runs now on 13mA able to swing more then 4V Class A into the 340 Ohm transimpedance resistor. This idle current is a liitle high for optimum noise match to the 220 Ohm resistors but the total noise is still around -140dB.

[Joachim Gerhard]

hat mir mein sohn rausgesucht. ich ziehe immer das an , was ich morgens neben meinem bett finde.

[john curl]

Syn08, is that a HP3563?

Joachim, please consider the overall picture, when it comes to phono overload. We have measured this over the decades, and truly it is often too little in many designs, BUT where does this extra output go, once it is generated? Yes, in the input attenuator. If you have significant overload problems, then you usually have too much forward gain. This can be reduced, often improving distortion performance.

[Joachim Gerhard]

Overload is simply not a problem in the MPP because i do the 75usec RIAA right in the first stage as part of the transimpedance gain structure, so gain at higher frequencies goes infinitesimal to zero, lowering distortion the higher the frequency is. Bass overload could be a problem with warped records but i use a rubber ring to lift the record and then press it down with a clamp. on the other hand my records (most) are in mint shape, i simply took good care buying and handling them. this little picture shows only my small analog lab where i check for very basic things. i have three other labs with laser scanners, accelerometers, extremely expensive microphones and a complete measurement suite for electronics measurements at infinitum. i have access to the latest Audio Precision and Rhode & Schwartz gear but does that make a better product ? Anyway i am very impressed Syn08 and you speak german too !!!! that´s not an easy language.

[syn08]

Originally Posted by Joachim Gerhard

and you speak german too !!!! that´s not an easy language.

Ich habe sieben Jahre Deutch gelernt, als ich ein Kind war.

[Joachim Gerhard]

in dem alter ist man noch lernfähig

[Joachim Gerhard]

Here is the circuit diagram of the DIY version of the MPP unbalanced head amp.
I will listen to it on the weekend and you will be very surprised how good the measurements look with this really simple topology. I can not hear any noise at the listening seat even at quite high volume and i live in a very quiet rural place. i will ask my son who has frequency extention to 20kHz ( i check that from time to time) if there is something unpleasant.