McIntosh C24 question

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I have a McIntosh C24 preamp and cannot find the input capacitance spec for the phono input anywhere.

What is the capacitance value?

I've got a Shure SC-35C cartridge on a RCA RP-190 wired in stereo and the treble isn't as good as it should be at least it doesn't sound as good as 45s did when played on my Pioneer PL-530 that I used to have.

If the input capacitance of the C24 is lower than 250pF (value that will give the flattest response from the cartridge) I'll add a capacitor to the input and see if that fixes it.

If the capacitance is higher than 250pF I need to figure a way to reduce it to 250pF.

If that don't work I'll need a better magnetic cartridge that tracks at 5 grams.
 
I think that circuit isn't connected to the phono input, but ill look at the full schematic and see.

I looked at the schematic and that circuit connects to the emitter of the first phono preamp transistor.

There is a 6.8pF cap from collector to base and a 39pF cap from base to emitter at the first phono preamp transistor. dont know if either of those affects the treble any.

Also what would the effect be of less than optimal capacitance for a phono cartridge?
 
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Assuming the SC35c DJ cartridge response is flat does not line up with my experience, so trying to Mod the McIntosh preamp would be a misguided attempt to fix what is not right to your ears. Try changing the cables from the RCA 45 changer to the preamp....look for lower capacitance cables that might be more similar to those used in your higher performance Pioneer table.
 
For the Pioneer I used some cheap component video cables as that is an easy way to get low capacitance audio cables.

When I first tested the phono I found the treble seemed a little better on my Rolls Belari VP-129 phono preamp if I used a pair of Monster Cable audio cables which have a little more capacitance than the cheapo RCA cables I was using.

I know it is the cartridge and all I would be doing is replacing the ceramic caps with better caps.

What I need is a cartridge that tracks at 5 grams, has good treble output and fits the headshell of an RCA RP-190 tonearm.
 
It has been my experience with these 50 year old Mac preamps that they need a good restoration before the response of the preamp in general, not even considering the phono stage, will be back to flat. Then checking the phono stage with a reverse RIAA signal will let you know how far out of spec the phono is.

If you are just trying to tweek your sound to spin 45s I think it would be best to attempt to do this outside the preamp but in the RCAs wiring to the preamp.
 
I do want the preamp to sound its best.

So far all electrolytic caps on the board have been replaced. The can caps were still good so I left them be.

I do need to check for and replace all out of tolerance resistors.

Now I have seen it mentioned online where ceramic disc caps are not the best choice for audio circuitry. Should I replace all the ceramic caps with mica (for the small values) and film (for the larger values) or will that affect the sound in a bad way?

I do know the preamp is a little noisy before the volume control so some transistors will have to be replaced, although I may just replace them all.

When I originally got the preamp one channel was dead due to a bad transistor on the output side of the board. I replaced it and the same one on the other channel with a transistor I happened to have two of and it worked.

I do not want to do any tweaking to make the 45 player sound better other than replacing the cartridge unless there is some readily available circuit that will improve the treble response of the cartridge and if so I'll install it in the phono itself.
 
Are you familiar with the idea of using a reverse RIAA network to check the linearity of the active RIAA equalization circuits of a preamp?
 
Unfortunately no.

Never have done that before.

Its not the preamp as the Pioneer PL-530 I had sounded good through it.
 
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The RIAA eq curve I believe was orginally derived from the old RCA eq parameters and is a passive design. By building a capacitive/resistive network the audio signal is altered to greatly cut the bass and moderately cut the midrange. This signal is then cut into the record mater etc. etc. Your phono preamp ampilfies the bass and midrange up to again be flat.....by building a reverse RIAA network you can drive a line level signal into your phono stage to check it's linearity and restore/repair it as necessary.

A handfull of caps and resistors are needed to build one, many plans are on the net and I believe Rod Elliot on his soundwest site details one as well as one of his phono preamps.

I actually have two that DOB built and gave me at two amps clinics I hosted back in the day.....I use them regularly to test and troubleshoot phono stages.
 
The RIAA eq curve I believe was orginally derived from the old RCA eq parameters and is a passive design. By building a capacitive/resistive network the audio signal is altered to greatly cut the bass and moderately cut the midrange. This signal is then cut into the record mater etc. etc. Your phono preamp ampilfies the bass and midrange up to again be flat.....by building a reverse RIAA network you can drive a line level signal into your phono stage to check it's linearity and restore/repair it as necessary.

On recording, bass is reduced and treble increased. Bass is reduced so that the waves will physically fit within the groove. On playback within the preamp's phono section, the bass is restored (increased) and the treble is reduced, also reducing surface noise.

Wikipedia: RIAA equalization

"RIAA equalization is a form of pre-emphasis on recording and de-emphasis on playback. A recording is made with the low frequencies reduced and the high frequencies boosted, and on playback the opposite occurs. The net result is a flat frequency response, but with attenuation of high frequency noise such as hiss and clicks that arise from the recording medium. Reducing the low frequencies also limits the excursions the cutter needs to make when cutting a groove. Groove width is thus reduced, allowing more grooves to fit into a given surface area, permitting longer recording times. This also reduces physical stresses on the stylus which might otherwise cause distortion or groove damage during playback.

A potential drawback of the system is that rumble from the playback turntable's drive mechanism is amplified by the low frequency boost that occurs on playback. Players must therefore be designed to limit rumble, more so than if RIAA equalization did not occur."
 
That Wikipedia post is incorrect, the RIAA eq concept was completely passive in design for encoding, decoding could be done with a single 12AX7 tube. There is no boosting or active amplification during encoding as it was designed.

The wiki post sounds more like a Dolby circuit description. Which is a active circuit.
 
In any case a passive reverse RIAA circuit allows you to verify your phono stages linearity and S/N ratio.

If you choose to experiment with changing your cartridges response you will know it is not the preamps fault for the faulty response.
 
Well the purpose of my post was to let people (who may not already) know the purpose and function of the RIAA curve - to overcome fundamental limitations of the vinyl medium. I was not trying to address how to implement circuits to apply or remove the RIAA curve.

The term "boost" is relative and in the functional description on Wikipedia and in the RIAA Spec, everything is relative to 1KHz and so on recording, frequencies above 1 KHz are boosted.

RIAA-EQ-Curve_rec_play.svg


How this is implemented (with passive or active circuits) is left to the designer and not what I am addressing above.

But for test, as you say, a passive (no transistors, tubes or power supply) inverse circuit can be built that would apply the recording curve to the test signal by attenuating everything below 20 KHz according to the shape of the RIAA recording curve. (So even though the whole signal is attenuated in the passive case, relative to 1KHz, 20 KHz is relatively "boosted".) In fact, since the test signal voltage is likely much greater than what the phone pre-amp sees from a phono cartridge, the losses from a passive test circuit are OK and in fact, the passive inverse circuit would probably apply attenuation across the whole band in addition to this curve to get the signal down to the appropriate levels.

In the preamp, the phono pre-amp section needs to be active because it must simultaneously physically amplify the low phono voltage to line voltage and apply the RIAA playback (compensation) curve to the signal.

The Dolby B system was designed to overcome the short-comings of cassette tapes and is active but also dynamic (I guess you could say) in that the compensation switches on and off "automatically" based on how loud the music is at any given time and does the opposite during playback. RIAA equaliztion is fixed with time and does not change in response to the music signal.

I am just trying to shed some light to interested readers by augmenting the correct information provided by c_dk.

Thanks
 
In any case a passive reverse RIAA circuit allows you to verify your phono stages linearity and S/N ratio.

If you choose to experiment with changing your cartridges response you will know it is not the preamps fault for the faulty response.

Agreed
 
Sitting in class back in the 70s I swore that if I heard this description from my engineering prof any more I would throw up......

but I know of no other word that can sometimes describe the simplicity of these old electrical circuits and their implementation........"elegant"
 
While listening today I decided to check why the left channel sounded a little lower.

I started with the Lafayette LA-375 amp since I had used Radio Shack 5% carbon film resistors to form a voltage divider to drop the level some since I feed the input directly to the amp stages after the volume/tone circuitry and with the preamp fed a 400Hz signal I noticed the left channel input to the amp was a little lower than the right channel.

I then measured at the input to the active crossover and noticed the same difference.

I adjusted the balance control until I got equal signal levels to the amp and the balance control has to be set at the 11 o'clock position.

Seems to indicate maybe a preamp issue. Perhaps I have some out of tolerance resistors.

I need to check where the imbalance is occurring.

Thing is I didn't notice it before the tree hit my house that I know of.

Perhaps whatever shock happened when the tree hit caused some issue with the preamp?
 
Just thought of something.

I could try the phono section of my Lafayette LA-375 and feed the record out jacks to the preamp input just to see how the cartridge sounds with it.

The LA-375 was modded so that the input signal from the C-24 feeds the amp after the volume/tone circuitry.
 
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