C37 phono mod?

[nowinowski]

I just hooked up a mcintosh c37 and was getting wicked hum through my denon 103 (.3mv) and step-up. Funny I thought, how very un-Mcintosh to have that much noise. Then I put on a record and the usable volume range is only 1-3 out of 100 - YIKES. 2 sounded great but was loud as crap. Next step i took out the transformer and hooked straight into the phono out. Sounds good and normal levels - nice between 20 to 40 out of 100. The simplest deduction seems to be that it has a MC phono - was that an option? Could it have been done as an aftermarket mod?

Also the preamp I am replacing is a almost identical MC-31v that I hated the digital volume control on. So i don't think it has anything to do with the basic setup of a mcintosh pre and peer amp (an adcom 200w driving jbl 150s)

 

[clinic-audio]

Hello

C37 McIntosh don't have MC Phono input , so your 300µV output level MC will not work correctly with it .
The Phono input is not adapted . Making a mod is not recommended . You better use MC pre-pre or a MC transformer .

Regarding C31 and C37 : you can't compare the both because one (C31V) is used 1 IC TC9153P for volume control , and C37 is used motorized volume potentiometer so you can't hear anything when you increase (or decrease) the volume level control
C37 is more quiet and better

 

[nowinowski]

I understand the specs. I am using a 1-20 step up with the denon 103 and it was so loud through the C37 that the volume control was basically unusable. This cart and step-up are the same i was using with the 31v which is supposed to have a similar amount of gain.

 

[clinic-audio]

What input did you use with your step up transformer ?

C31V Phono input idendical in C37
switching by FET in C31V , relays in C37 but this do not make your MM Phono input working with a MC cartridge
a PRE-PRE will work much better in high level inputs

 

[nowinowski]

What input did you use with your step up transformer ?

103 > step-up > phono on C37

same configuration I have used with a number of pre-amps

 

[clinic-audio]

Sensibility of your Phono input is 2,5mV with C37 and 2mV with C31V
What voltage get you at the output of this transformer ?

 

[nowinowski]

It should be 6mv into the C37 phono

 

[62caddy]

Aren't phono preamplifiers supposed to be connected at one of the line level inputs? Ie: AUX, TAPE, CD etc.

 

[nowinowski]

the phono stage is built into the C37 - that is the part i'm trying to figure out if it could have been modified to a higher gain MC by Mac or aftermarket.

 

[62caddy]

Your original post mentioned "Denon 103 and step-up". What's the "step-up" device?

 

[nowinowski]

Sony Ha-t10 step up transformer

https://www.vinylengine.com/library/sony/ha-t10.shtml

 

[62caddy]

Thanks I understand now.

Perhaps if you posted pictures of the unit with the case removed and Patrice determine whether modified.

 

[nowinowski]

I'll post a photo tmrw. Heard back from the shop the the previous owner had a low output MC and had used a resistor to modify the phono stage. Does that make sense?

 

[nowinowski]

if anyone is familiar with these unit i would love if you could tell me what has been modified and to what effect. the place i bought the pre-amp from found out the previous owner had "changed a resistor" because they had a low output mc cartridge. Trying to figure out what exactly is going on and what i should do to get the best sound.

 

[clinic-audio]

Hello

if anyone is familiar with these unit i would love if you could tell me what has been modified and to what effect.

I am , and I can tell you that every components on these boards in PHONO input are 100% original ! none has been replaced or added

 

[twiiii]

From what I read Adcom amps have 6 db more gain than Mac amps and pre-amps use. Then the step up phono transformers have an additional 6 db more gain than is optimal, plus the Denon cartridge has a db or so higher out put than necessary. So thats anywhere from 12 to 13+ db more gain in the system. Thats quite a bit and creating your issues. I would recommend replacing the adcom amp with a Mac amp with input sensitivity controls. 7200 if you want a direct coupled amp, or a 7150 or 7300 for amps that use autoformers. You could also look for more modern pre-amps where the input sensitivity of each input can be regulated any where from 6 to 10 db up or down.

Your learning that it is sometimes its unwise to use amp of one manufacturer with a pre-amp of another.

 

[welcomdmat]

twiiii has great info that I kind of wish wasn't true. I mixed a Mc amp with another preamp that made sense on paper. It sounded really good -- until I went Mc/Mc. I have mixed very different generations even.

I would never argue that cross brand components won't sound great together. I have found that same brand components do sound great together.

 

[62caddy]

Never a bad idea to verify compatibility between units, ie: voltages, impedances etc.

 

[TSmith8605]

I'll post a photo tmrw. Heard back from the shop the the previous owner had a low output MC and had used a resistor to modify the phono stage. Does that make sense?

I can't help you reverse engineer the change but you could have the pre-amp tested to at least verify the change as follows.

Both the C31v and C37 are spec'd at 60 dB of gain in the phono section (a voltage factor of 1,000) so if you put the C37 on the bench and feed 2 mV RMS (at 1000 HZ to avoid the RIAA curve), you should see 2 Volts RMS out of the phono stage (tape monitor output). If you see a much higher voltage, then the C37 has been altered and is not operating to spec. You can do the same with the C31v to see if the output is too low.

This will at least verify that the C37 has been changed which seems likely given what you describe and what the shop says.

This test could provide a basis for returning the pre-amp to the shop as being tampered with and out of spec.

You could just operate the C37 as is without the step up transformers; as you say it seems to work OK and would avoid any noise, distortion or coloration added by the Sony transformers.

But I would want the pre-amp to work correctly. Some day you might want to sell the pre-amp or use a MM cartridge. Also, the higher-gain modification may (or may not) have compromised other performance parameters.

It seems like reverse engineering someone's undocumented modification may be difficult. You might see if your shop can contact the prior owner to see if they can tell you exactly what was changed. Otherwise, someone will need to go through the phono boards and compare every resistor (or everything) with what's in the schematic - perhaps looking for signs of different types of resistors or redone soldering. It does not seem like you're getting the answer you need here but this may not be easy.

Good luck.

 

[ron-c]

60dB of gain is all the DL103 needs.
Ron-C