improving the Mcintosh C22

[Craig405]

What do people think of this classic preamp?
and how could it be improved?

I have put in a tube rectifier (5Z4) and a tube regulated supply so far on mine (VR150's and VR105's) and run the heaters from DC, it sounds good.
Although at full output i get 0.3v pk pk supply ripple in the form of my music is this acceptable?!

I dont know much about tube circuits yet but is there any advantages changes in topology can give like SRPP stages, active loads?

Anyway its a really easy preamp to build

Cheers
Craig

 

[SY]

at full output i get 0.3v pk pk supply ripple in the form of my music is this acceptable?!

Are you saying that you see 0.3V of signal on the rail? Or do you see 0.3V of ripple on the output?

 

[Frank Berry]

"Anyway its a really easy preamp to build"

I assume that you're saying that it's not a "real" McIntosh C22 but rather a knockoff kit.

If I were building this copy, I would be inclined to find a Service Manual for the C22 and start by building it exactly as McIntosh built it.

 

[Craig405]

I measured my supply rail with a scope and saw 300mV, i didnt check the output.

 

[SY]

300mV is not a huge amount of ripple, but I would ideally want it lower. The important thing is how much is appearing on the output.

 

[Craig405]

Frank

Yea it is not the whole circuit just the one in my diagram, i didnt put in the phono stage and the tone controls etc.

It is not a kit though, i just put one togeather because i needed one and it looked easy.

SY
That sounds very sensible i will get that measured very soon and post what i find.
Im guessing its ok because i saw no interference on the signals from both both channels (they both run from the same regulated supply) when testing a while ago.
I was just disappointed to get any ripple after putting my nice regulators in.

cheers
Craig

 

[SY]

Whoa! If you've got regulators, 300mV is a LOT of ripple. My preamp B+ regulator, a very simple 2 transistor job, gets me below 0.1mV. I'm not familiar with tube regulator performance, but I'd be astonished if you can't do better than that.

Now that said, what matters is (again) what's at the preamp's output.

 

[cerrem]

The ONLY 12AX7 I would use for Hi-End audio is a Telefunken....
I don't see the point to using 2 Gain stages before the follower..then using a TON of feedback to bring the gain down...
I would get rid of the two coupling cap that are in series that feed the grid of the follower....I would go DIRECT COUPLED...then hook up the feedback to the output of the follower...
As for the B+...make it CLEAN and FAST as possible...

Chris

 

[Craig405]

SY
yes i know i was suprised too it is definatly an issue. Ill try swapping the regulators about.
The supply before them is adequate enough i think 22uf then 20H then another 146uf of parallel caps.

Cerrem
Thanks for your input
I currently am using a selection of old triodes (12AX and 12AT as buffer) Consisting of Mullards, Brimar and one Hitachi! all are quite old already and probably arent performing their best.

I will read into direct coupling, this involves letting DC onto the buffers Grid?
I was actually thinking of changing my coupling cap values in order to get a lower frequency response to match my power amp, (-3dB at about 8HZ) but direct coupling might do the job.


Cheers
Craig

 

[kmtang]

Hi Craig

I just ran the PSU program. With just 22uF-20H-146uF CLC pi filtering, the ripple should only be 3mV. Are you sure that your measurement is correct?

With 300mV ripple, the hum noise would be so high that you would shut the damn thing down immediately.

I believe the MC C22 is a good performer. It is so famous compared to the Marantz M7. However, the Conrad Johnson's PV6 really caught my eye.

Johnny

 

[Craig405]

Ok i have just remeasured the rails and outputs.
(i didnt use 310V and 290V because i didnt want to punish my tubes so i used 300V and 255V)

Without any signal supply rails measured:
The 300V supply = 302V @ 25mV ripple
The 255V supply = 265V @ 25mV ripple
The signal outputs gave about 20mV ripple each.

With signal output from preamp of 10V pk-pk i get:
1.1V pk-pk on the 300V rail !
0.25V pk-pk on the 255V rail !

The resulting crosstalk measures 50mV on one channel and 120mV on the other.


Well i feel worse about my preamp now, it clearly needs more attention!

Cheers
Craig

 

[SY]

Look at grounding. I'll bet that most of your issues are there.

 

[Craig405]

I have re-soldered every ground and measured them with an accurate meter and they all measure under 0.05 ohms to the star point.

Things have improved slightly , i now get 20mV ripple on each supply.
Still a bit much but i guess tube regulators arent as good as transistors, i reckon some of this is from the heaters which are run from a less than flat DC supply, i might add more caps to it and even it out.

The crosstalk measures -40dB on one channnel and -46dB on the other, which is much better, i guess i just need to improve my layout to optimize this.
I dont know if this is considered acceptable by others?

Thanks
Craig

 

[SY]

One point often overlooked (I'm guilty of that!) is pickup of noise from common mode sources in the heater line. So adding caps is nice, but won't solve this- it will take a couple of caps and chokes. There's a useful discussion of this in Morgan Jones "Valve Amplifiers."

Your crosstalk is pretty high. It might be worthwhile to check it at 1KHz and 10KHz to see if it's capacitive.

 

[kmtang]

I guess there's something wrong with the power supply. I could hardly imagine the amount of ripple is that high in the B+ rail.

I am wondering what kind of bypass capacitors you are having. Try use better ones like the Black Gate.


Johnny

 

[Craig405]

Its a possibility I am not exactly using high quality capacitors, red things from RS.

but they were brand new so shouldn't be leaky. I might think of upgtrading them once i get everything quite and stable but i definatly need a new set of tubes first!

I just obtained a really cool signal generator today so i can test for crosstalk at a range of frequencies.
It is a tube jobbie and will do 1hZ to 120KHZ at 20V pk-pk square or sine i was very impressed for 10 UK pounds it is noiseless too i cant see a flaw in the wave at full output with my scope on gain x10.

Cheers
Craig

 

[Craig405]

I think i just solved the supply noise issue, I just measured my circuits ground from my scope probe and there is 20mV of noise from the mains, with amp completely off.
Also with just the heaters on it remains at 20mV.
That suggets to me that my scope is detecting its own power supply ripple?.

I do not have a mains earth ground yet it just goes to the CT of my transformer ( i know this is very dangerous and stupid) would that help noise?

My heater ground also goes to the same star point as my circuit is there a better configuration that this.

Craig

 

[anatech]

Hi Craig,
Time for a better 'scope. Anyhow, you are measuring peak to peak values, this okay for supply ripple. Normally audio specs are in RMS units, like hum and noise.

When you can afford it, pick up a new 20MHz 'scope. There are cheap these days. You must be very careful you don't blow them up as they are grounded and most older gear has a "floating" ground.

So how does it sound now?

-Chris

 

[Craig405]

hi anatech
Yes you are right a better scope would definatly be on my agenda if i werent going to have access to a university lab soon .
As to sound i cant yet comment as my Quad 405 power amplifier is still in the testing and tweaking stage (parasitic oscillation )

SY
I have just tested the crosstalk at 100hz,1Khz and 10Khz and 20Khz it measures :

Pre volume pot:
(pot is inserted between last gain stages anode and buffers grid)
100= -52dB
10K= -52dB
10K= -67dB
20K= -64dB

output of amp:
100= -46dB
1K= -44dB
10K= -30dB
20K= -24dB

This makes it quite clear where a big problem lies, i have long wires slumped over the entire circuit unshielded going back and forth from the pot ..whoops.
I think another solution to the crosstalk is to quarter the gain of my tubes as they are capable of 20V peak output and i only need to drive my Quad!!.
I might also reduce input impedance from 1M to 100K or so.

Cheers
Craig

 

[kmtang]

I built the 6SN7 line amp 2 days ago. I spent quite some time figuring out how to kill the hum.

The trick I found is that it needs to raise the filament to about half of the B+. Also, a bypass capacitor of 2.7uF is needed to be connected at the bottom leg of the potential divider.

It is now pretty quiet and I could enjoy the music.

Johnny