McIntosh C28 — What’s not to like?

[cosmo61]

The biggest improvement came from replacing the electrolytic capacitors and the output transistors. The original transistors were beginning to fail, and the amplifier sounded a little too soft.

I only replaced resistors that were out of specification. Later, I installed NOS ERO MKT polyester capacitors with the correct footprint. It sounds very good now.

MC2105 – Ringing/early clipping after output board upgrade with modern transistors – no visible oscillation

 

[twiiii]

C28's are known for their trouble some trim controls, rotary switches and the poor tracking of the main volume control. The metal panel with the low level rca inputs has grounding issues. There was a mod to lower the crosstalk of the low level inputs also. The phono inputs are very warm and smooth sounding. I got rid of mine because Davie Obrien got tired of fixing it every year at Mcintosh Clinics we had. He told me to get a C29 and I did ordering it from our Mac distributor the same day. I had it over 25 years and the C29 never gave me any problems. I now have a C34 for the stereo system. Love the more advanced graphic controls. That reminds me the steps of the tone controls of the C28 and C29 were both to large, which irked me also. I would have a C40, but don't have the room in my cabinet.

 

[cosmo61]

Absolutely, it has plenty of character.
The C29 has better specifications, but it also uses op-amps and has a somewhat different design philosophy.
The C34 looks great—it heavily borrows from the classic C29 design language.

 

[cosmo61]

I started looking a little at the phono stage of my C28 today. The weather has been too hot to do much work on it.

One simple test I tried was to short the unused phono input with an RCA shorting plug. In my case it did not really change the noise level.

Still, I think it is a useful diagnostic test. It can help show whether the turntable cables, phono wiring, or an open unused input are picking up noise. If the noise level changes when shorting the unused input, or when connecting/disconnecting the turntable cables, that may point to noise pickup outside the phono stage.


The pictures show the shorting plug I used.

 

[cosmo61]

Made a little more progress on the C28 despite the hot weather.
I finally finished building my transistor noise test jig. The screenshots show a couple of example measurements. The absolute dBFS values are not calibrated, but that’s not important for this project—the jig is designed for relative noise measurements, and most importantly, the results are repeatable.

The goal is simply to compare transistors under identical conditions and select the quietest ones for the phono stage.

Picture 2 is the souncards noice floor around 130 in my setup.

More to come as I continue testing different devices and working through the C28 RIIA.

 

[cosmo61]

A quick follow-up after some more testing.

I compared several BC549, BC550 and KSC1845 transistors using the same setup and test conditions. So far, the KSC1845 has consistently proven to be the quietest in my jig. It also seems to be a little more stable at very low frequencies (around 20 Hz).

Since this is a relative noise comparison, I’m mainly looking for repeatability and comparing devices under identical conditions rather than absolute dBFS values.

I ended up selecting KSC1845s with an hFE of around 450–500, which is in line with the gain range of the original McIntosh Noise-Select transistors, i guess

The KSC1845 measured about 4 dB lower than the BC549C/BC550C samples I tested in the 20–640 Hz range. I haven’t compared it directly with the original McIntosh 132-105 Noise-Select transistors, as I don’t want to risk removing the original parts just for a noise comparison.

One thing to keep in mind: the KSC1845 has a different pinout than the original Q1/Q2 transistors in the C28, so it must be installed accordingly.

 

[cosmo61]

Final update: I’ve now measured the actual C28 phono stage with the inputs shorted. Both channels are very close, measuring about 169–171 µV RMS at Tape Out (20 Hz–20 kHz), so the phono stage is essentially within the original McIntosh noise specification.

The KSC1845 still won my little transistor beauty contest, measuring about 4 dB quieter than the BC549C/BC550C samples in the jig.

But looking at the actual C28 measurements, I doubt replacing the original 132-105 Noise-Select transistors would gain much in the real circuit.

So the originals are staying put. If I want a really quiet phono stage, I’ll just use my Pass Pearl II and let the C28 continue being a C28.

Picture 2 shows the AUX input at 14.8 µV RMS — really quiet

The 169–171 µV RMS at Tape Out, equivalent to roughly 0.88–0.89 µV input-referred noise — comfortably within the original McIntosh <1.2 µV spec.

 

[seacliffe301]

I loved the C28 when I had it, but the C32 was still current when a used one turned up on my Mac dealers used shelf due to a divorce (meaning cheap). I had to grab it. It was a game changer, whatever I loved about the C28 I loved more about the C32 with phono preamp being the most obvious improvement. I was using a 2505 at the time, they mated together great.

That being said, I did love that preamp. If your's is healthy and it sounds great with your 2105 (which of course it would) stick with it. If you decide to move on but want to stay vintage Mac I would highly suggest a C32 or C33. That could be a forever system with your amp.

 

[SaturationPt]

There are better and worse things about each evolution, depending on your preferences.

Preamps from the early ‘70s vs late ‘80s really don’t bring much more or less in money, but reliability certainly has improved as well as simply the age of the components you’re buying.

I had a C28 years ago, very much liked everything about it. When I moved to newer amplifiers however, … *while I’m at it” LOL. Eventually my system moved to the black knobs, then to wide-format, XLR/Balanced, remotes, blah-blah. The newer gear certainly is easier to maintain and compared to early ‘70s gear quieter FWIW. However that doesn’t make it more “musical” and for a while you gave up a second Phono input for video and DVD inputs, for TT fans that is certainly a downside.

I’m very happy with my current choices, as I was with every generation of McIntosh preamp I’ve owned. No bad choices as long as they’re properly maintained and serviced and mind that volume pot, all should be great.

 

[cosmo61]

This is pretty much how I look at it.

I am a retired technician and worked with electronics for around 30 years, so one of the things I really appreciate with the C28 is the old-school craftsmanship.
I know it is not the “best” design by modern standards, and I also understand that the C29 was a fairly big technical update. But the C28 still belongs to that older McIntosh design tradition, and that is exactly what I like about it.

For me, part of the charm is that it is a kind of time capsule.
I am not trying to turn it into a modern preamp. I am trying to preserve and recreate what that system was supposed to sound like in the 1970s.
That is also why I measure it carefully. The phono stage is not the quietest one ever made, and yes, it could probably be improved with more grounding work, rerouting of the phono wiring, better separation and so on.
But McIntosh clearly did not think that was necessary at the time, and changing too much would also change the character of the unit.

A good example is my Thorens TD-124. It is a wonderful old design, and mine has been fully restored.
With a really quiet modern phono stage it is basically silent in use, but there is still a very small amount of rumble from the idler wheel system. You do not hear it while music is playing, maybe only a little between tracks depending on the volume.
Many modern turntables will measure better, no question. But when it comes to speed stability, punch and drive, not many of them give the same feeling.

I think the same applies to vintage preamps and amplifiers. They have their weaknesses, but also their strengths. Do those weaknesses automatically mean they sound worse? I do not think so.

I also own some reference amplifiers that beat my old McIntosh setup in almost every measurement. But do they always sound better? Sometimes yes, sometimes no. It depends on the music, the system and what you are listening for.
Sometimes a very modern and analytical system can be impressive, but maybe not always the most enjoyable.

So that is my view: I respect the measurements, and I use them to make sure the equipment is healthy.
But in the end, I also want to preserve the character and the musical experience of the old McIntosh system. That is the fun part for me.

 

[seacliffe301]

This is pretty much how I look at it.

I am a retired technician and worked with electronics for around 30 years, so one of the things I really appreciate with the C28 is the old-school craftsmanship.
I know it is not the “best” design by modern standards, and I also understand that the C29 was a fairly big technical update. But the C28 still belongs to that older McIntosh design tradition, and that is exactly what I like about it.

For me, part of the charm is that it is a kind of time capsule.
I am not trying to turn it into a modern preamp. I am trying to preserve and recreate what that system was supposed to sound like in the 1970s.
That is also why I measure it carefully. The phono stage is not the quietest one ever made, and yes, it could probably be improved with more grounding work, rerouting of the phono wiring, better separation and so on.
But McIntosh clearly did not think that was necessary at the time, and changing too much would also change the character of the unit.

A good example is my Thorens TD-124. It is a wonderful old design, and mine has been fully restored.
With a really quiet modern phono stage it is basically silent in use, but there is still a very small amount of rumble from the idler wheel system. You do not hear it while music is playing, maybe only a little between tracks depending on the volume.
Many modern turntables will measure better, no question. But when it comes to speed stability, punch and drive, not many of them give the same feeling.

I think the same applies to vintage preamps and amplifiers. They have their weaknesses, but also their strengths. Do those weaknesses automatically mean they sound worse? I do not think so.

I also own some reference amplifiers that beat my old McIntosh setup in almost every measurement. But do they always sound better? Sometimes yes, sometimes no. It depends on the music, the system and what you are listening for.
Sometimes a very modern and analytical system can be impressive, but maybe not always the most enjoyable.

So that is my view: I respect the measurements, and I use them to make sure the equipment is healthy.
But in the end, I also want to preserve the character and the musical experience of the old McIntosh system. That is the fun part for me.

Nothing wrong with that. One has to remember that a C28 and 2105 combo was once the flagship system for Mac, add an MR77 tuner and you've completed the package. I for one still feel that vintage McIntosh is still an outstanding platform to enjoy music with and in my case my preference. One doesn't (or shouldn't) compile a system based on specs alone, it has to compliment the listening experience. If that does that for you, you have succeeded. Everyone of the components (with the exception of MPI-4) of my main system have been with me for decades despite improvements in specs and functionality, simply no desire to move on from any of those components as the experience still works for me. About a year and a half ago I swapped out a MAC 1700 for a 4100 not because of performance issues or SQ, I simply needed more line level inputs. The 1700 was still a joy to listen to and for the time being it is in reserve waiting to be re-purposed. Congrats on your new system, BTW what speakers are you using.

 

[cosmo61]

Thanks!
Yes, that is pretty much how I feel about specs too. They are useful, but they are not the whole story.

The speakers I use are Fritz Carbon 7 SE. They are a bit special, somewhere between the Mk1 and Mk2 version, with the ScanSpeak D3004/6600 AirCirc tweeter and the 8545-01 woofer.

Fritz told me the AirCirc tweeter is every bit as good as the one used in the Mk2, so they’re really more of a special variation of the later Carbon 7 SE.
They work really well with the C28 and MC2105.


They work really well with the C28 and MC2105.

I also use a REL T/5i subwoofer, but only to support the lowest bass.

What speakers are you using with your system?

 

[seacliffe301]

Thanks!
Yes, that is pretty much how I feel about specs too. They are useful, but they are not the whole story.
The speakers I use are Fritz Carbon 7 SE. They are a bit special, somewhere between the Mk1 and Mk2 version, with the ScanSpeak D3004/6600 AirCirc tweeter and the 8545-01 woofer. They work really well with the C28 and MC2105.
I also use a REL T/5i subwoofer, but only to support the lowest bass.
What speakers are you using with your system?

That must sound pretty good. What powers the sub or is it self powered?

My main system has Altec-Lansing Flamenco's circa 1968, they are the predecessor to the iconic Model 19 and brother to the Valencia's. I'm a lifelong Lansing guy, take a look at my equipment profile, you'll see all the rest of my systems are built around JBL's. Looking at a 50 year window of acquiring this type of stuff I found the McIntosh/Lansing combination to be quite a good fit, those Altec's have been with me since 1977. I've even got a Thorens in the mix as well LOL.

 

[cosmo61]

That must sound pretty good. What powers the sub or is it self powered?

My main system has Altec-Lansing Flamenco's circa 1968, they are the predecessor to the iconic Model 19 and brother to the Valencia's. I'm a lifelong Lansing guy, take a look at my equipment profile, you'll see all the rest of my systems are built around JBL's. Looking at a 50 year window of acquiring this type of stuff I found the McIntosh/Lansing combination to be quite a good fit, those Altec's have been with me since 1977. I've even got a Thorens in the mix as well LOL.

Thanks! The REL T/5i is self-powered. I only use it to fill in the very bottom end, so it doesn’t really draw attention to itself.

Those Flamencos must be a great match with McIntosh, especially after all those years. I can see why you’ve stayed with the Lansing sound.

And yes, I’ve got a Thorens too.
Funny how this old gear seems to find its way into the same systems.
Nice setup you got.

 

[SaturationPt]

This is pretty much how I look at it.

I am a retired technician and worked with electronics for around 30 years, so one of the things I really appreciate with the C28 is the old-school craftsmanship.
I know it is not the “best” design by modern standards, and I also understand that the C29 was a fairly big technical update. But the C28 still belongs to that older McIntosh design tradition, and that is exactly what I like about it.

For me, part of the charm is that it is a kind of time capsule.
I am not trying to turn it into a modern preamp. I am trying to preserve and recreate what that system was supposed to sound like in the 1970s.
That is also why I measure it carefully. The phono stage is not the quietest one ever made, and yes, it could probably be improved with more grounding work, rerouting of the phono wiring, better separation and so on.
But McIntosh clearly did not think that was necessary at the time, and changing too much would also change the character of the unit.

A good example is my Thorens TD-124. It is a wonderful old design, and mine has been fully restored.
With a really quiet modern phono stage it is basically silent in use, but there is still a very small amount of rumble from the idler wheel system. You do not hear it while music is playing, maybe only a little between tracks depending on the volume.
Many modern turntables will measure better, no question. But when it comes to speed stability, punch and drive, not many of them give the same feeling.

I think the same applies to vintage preamps and amplifiers. They have their weaknesses, but also their strengths. Do those weaknesses automatically mean they sound worse? I do not think so.

I also own some reference amplifiers that beat my old McIntosh setup in almost every measurement. But do they always sound better? Sometimes yes, sometimes no. It depends on the music, the system and what you are listening for.
Sometimes a very modern and analytical system can be impressive, but maybe not always the most enjoyable.

So that is my view: I respect the measurements, and I use them to make sure the equipment is healthy.
But in the end, I also want to preserve the character and the musical experience of the old McIntosh system. That is the fun part for me.

The grounding updates are not a change in sound, I certainly appreciate your desire to keep the original sound of the unit. The grounding updates (including McIntosh Service Bulletins to address) were to reverse degradation in sound and increased noise due to the crimped shields/drains on the shielded cables where the connection becomes corroded and changes the sound / creates noise.

Same with the input RCA jacks that are crimped to the chassis for their ground path.

Time and humidity takes its toll, but I agree that as-designed and in “perfect” original condition they sound excellent!

 

[cosmo61]

The grounding updates are not a change in sound, I certainly appreciate your desire to keep the original sound of the unit. The grounding updates (including McIntosh Service Bulletins to address) were to reverse degradation in sound and increased noise due to the crimped shields/drains on the shielded cables where the connection becomes corroded and changes the sound / creates noise.

Same with the input RCA jacks that are crimped to the chassis for their ground path.

Time and humidity takes its toll, but I agree that as-designed and in “perfect” original condition they sound excellent!

I agree 100%.

I already liked the c28 before, but after the service it was on another level. At that point I really did not want to change the circuit anymore. I would much rather make sure the original grounding, shielding and connections are working as intended and leave the actual design alone.
That is really the balance I am looking for with vintage gear: fix what has aged, but keep the character that made it special in the first place.

 

[cosmo61]

Update

I have done some further measurements on my C28.
With 250 mVrms into AUX and approximately 2.5 Vrms from the Main Output, I now measure:

• Left: 0.00408% THD
• Right: 0.00475% THD

Both channels are very quiet and stable, and the unit sounds excellent.
The only thing I am still looking into is the frequency response. Tape Out is essentially flat, while Main Out shows a gentle HF roll-off:

• about −0.2 dB at 5 kHz
• about −0.45 to −0.5 dB at 10 kHz
• about −1.35 to −1.6 dB at 20 kHz

The original C28 specification is +0/−0.5 dB from 20 Hz to 20 kHz, so the only clear deviation I am seeing is in the top octave.
Both channels show almost exactly the same curve, which makes me think this is something systematic rather than a random component failure.

The preamp sounds excellent, so this is not a major fault — mostly curiosity and an attempt to understand why the Main Output rolls off while Tape Out remains flat.

 

[SaturationPt]

The Tape Out should be straight-through, the Main Out goes through the tone section and controls, Volume, Balance. Likely something in that section is changing the signal slightly.

 

[cosmo61]

Thanks, that is also where my measurements seem to point.
Tape Out is essentially flat, while both Main Output channels show almost the same gentle HF roll-off. I’ll take a closer look at the tone/high-level/filter section and also double-check the capacitor values that were replaced there.

 

[cosmo61]

Final update on my C28:
I think I’m finally done chasing this one.

I made a lot of measurements through the signal path, and in the end I could not find one bad component responsible for the HF roll-off. It looks like much of the remaining deviation was simply coming from aged/oxidized contacts in the switches and control paths, with several very small losses adding together.
After a thorough cleaning, the frequency response improved quite a bit. At 20 kHz it went from roughly −1.3/−1.4 dB to about −0.73 dB, and 10 kHz improved from about −0.45 dB to −0.27 dB.


So after roughly 50 years of use, it is now very close to the original McIntosh +0/−0.5 dB, 20 Hz–20 kHz specification. I’m more than happy with that and I’m not going to start replacing original parts just to chase the last few tenths of a dB.

The distortion numbers are also excellent. At about 2.5 Vrms output / 1 kHz, I measured around 0.0041% THD.
The C28 sounds excellent, measures remarkably well for its age, and I think this is a good place to stop. Time to enjoy it instead of measuring it.