Shure M64 phono/tape preamp
Comments
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A detail photo of the three position selector switch for changing the preamp to phono, tape or flat response.
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I've been doing some electrical measurements and preliminary listening with the Shure. The first thing I find is that the left and right channels are extremely well matched. In fact I've never measured any device that ended up with the same output levels for both channels. At first I thought my H-P meter was stuck, but no the meter's needle only moved the thickness of the indicator when switched between the left and right. I carefully matched all of the transistors for gain, going through dozens to find the best ones so that might be why. Also there aren't any variable resistors or potentiometers to cause tracking differences between the channels.
The phono frequency response using the reverse RIAA circuit was within 0.5dB from 20Hz to 20kHz and only down 1.4dB at 32.0kHz. The averaged THD of both channels was 0.04% at 10kHz, 0.53% at 100Hz and 0.16% at 1kHz for phono. For tape the THD was 0.12% at 10kHz, 0.15% at 100Hz and 0.02% at 1kHz.
All within the original Shure specifications.
I measured the IM distortion (SMPTE method 4:1 ratio) as 0.1% for left channel and 0.09% for right channel. The tape gain was 36dB (Shure specified 37dB) in their manual.Post edited by SeleniumFalcon on -
I started out listening to the Shure as a phono preamp, substituting it for the vacuum tube unit and my general impression was that some high frequency change happened. I'm working on a method to be able to measure the frequency response from a test record using an Ortofon test record and a chart recorder, but I'm not there yet. I think the cause of the tilt is that I'm using a moving coil cartridge and step up transformer, so I believe the 50k loading of the Shure (along with 330pF) is being reflected back to the cartridge and effecting its loading.
In any event I'm most interested in using the Shure as a tape playback device. I connected it to the Crown and was very favorably impressed. The output is not as high as the de Havilland but it is noticeably quieter (I believe due to the battery power supply). After adjusting for the volume differences I went back and forth between the two listening to Kind of Blue on a Columbia prerecorded tape. The de Havilland has a palatable coloration, especially in the middle range. Both Coltrane's and Cannonball's saxophones were rendered larger than life (so I imagine) more forward in the stereo mix and I was aware of the echo that was added during the recording (a kind of ringing sound that haloed the fundamental saxophone sound). The Shure, on the other hand, was better balanced in the right and left channel which might lead to a feeling of a wider sound stage, but is actually just a better channel to channel level matching. I could easily detect the drummer's brush work (which some listeners have misslabeled as "swishing noise") on two cuts.
More fun ahead!
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Shure has provided a way of increasing the gain of the preamp by either 3, 6 or 12dB by paralleling resistors to R13 and R14. I chose to do a 6dB increase by adding 1.0k resistors which increased the gain from 36dB to 42dB.
I also added a toggle switch to the battery compartment making on and off more convenient. -
One of my most telling tapes is a 1961 London recording of Benjamin Britten's Peter Grimes. This recording is famous for being one of the most spaciously accurate ever made. This photo shows how the performers were placed at specific locations on the stage indicated by designated areas.
Rather than give everybody a separate microphone, as would be done now, an simple array of overhead microphones remained stationary and the singers would be told to move forward or backward to get a proper balance. If the singer were particularly loud he or she would be told to move one square backward. The same of the reverse, quieter singers would be moved forward to create a balanced sound. Scenes involving character movement were carefully rehearsed so that a sense of people actually moving around the stage was created.
The result is a breathtaking sense of realism, you will hear a wooden stage with people on it. The Shure combined with the Crown made this completely real. -
Hi @SeleniumFalconSeleniumFalcon wrote: »....
...so I ordered some more modern versions of these transistors getting complimentary NPN/PNP pairs.
sorry if I have missed it in the thread but what transistors did you end up replacing them with ? Can you share the part#s ?
Thanks!
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I checked on my project notes and I ended up with the Central Semiconductor's 2N5087 and Fairchild TIS97. I tried lots of substitutions, including European versions, and looked for linearity advantages and couldn't find any. I must have looked at over 50 transistors very methodically. I bought multiple examples of each one and carefully gain matched them until I found two closely matched pairs. I'm not sure where I found the Fairchilds, maybe an online vintage parts supplier?
I hope this is helpful. -
Thank you for responding!
I was going to explore modern transistor options that are specifically categorized as 'small signal audio amplifier' purpose but sounds like you have already gone down that path and havent found anything that offers a huge improvement over the stock tis97 and 2n5087.
I had chosen the KSC1008 and its complementary(pnp) but I will hold off on that for now.
Did you stick with the same power supply ? I am using mine with 4 9v batteries (36v) and I do think it made an appreciable improvement in the sound, so I might turn into a diy'ed external psu of higher (~34v) supply rail.
edit: and one more improvement that I felt was worthwhile was to reduce the input loading of the preamp a little bit. Stock is ~55k, slightly lower seems better takes the edge of the highs. I did this by dropping the 180k base bias resistor to 150k. Try it if you are up for it. -
I'm glad the information was helpful. I'm using the companion Shure battery power supply with three 9 volt batteries. It seems very quiet and dynamic. I use the Shure in the "flat" setting for some additional gain from the tape preamp I built and the combination sounds really good once I balanced the outputs and inputs. I compared it to a vacuum tube based combination and went with the Dundovic/Shure combination, but different playback heads might reverse the selection. The Shure company certainly knew their way around microphones and phono cartridges.
