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It looks like I will be the new owner of a very clean MC2505 SS Stereo amp. This early low powered Mac has big blue meters and a gain control for each channel. For the time being I don't plan on using a preamp. I know the amp will function using the gain control.
My question is how will the sound suffer not using an active preamp?
I do plan on a MX113 (first choice)or a C26 or C28 preamp down the road.
The amp will be used for our living room TV. We listen to concerts, live music broadcasts and enjoy movie soundtracks. An FM tuner would be used, especially for SEAHAWK games.
Follow Ups:
To me, the MC2505 with a MX110, MX112, or MX113 is THE classic McIntosh system. Stacked in their wood sleeves just look perfect.
Don Brian Levy, J.D.
Toronto ON Canada
Over the years I've read many who have shared enjoying the first generation Mac gear. I especially appreciated your detailing it Brian.
I so clearly remember seeing it in the PX in SE Asia but it was too expensive. I've always had blue meter fever so now after all these years I have scratched that itch.
Go ahead, blame me for everything; my wife does. :)
Don Brian Levy, J.D.
Toronto ON Canada
Yep, I got it the first time my dad took me to a store that sold Mac gear. Like many others, I couldn't afford it. But as a less expensive alternative, I did buy the Dynaco Stereo 150 kit (ca. 1978) with the blue meter option, it was good enough for me. Alas, now my ex-wife has it, and it still works great.
Over 20 years ago, my wife and I visited Audio Classics in Binghamton (back when they were in the basement of the post office). They had an MC-7270 power amp that I was coveting. My wife's only question - "Those meters, do they glow blue or do they glow green?" I said, "They glow blue." She replied, "Then you can buy it."
The guys at Audio Classics were great. And that amp, and my wife, are with me to this day.
The MC2505 sounds great. I found an MX113 that looks great should have that this weekend! Old School.
Eli, my 12 year old son and I are going to build the passive as you suggested anyway. It will be a neat project we can both do. Thanks for taking the time to detail it for us..... Dan
Family-philia!!
E
T
Full power O/P of 50 WPC is obtained with 0.5 V. of drive. You don't need a preamp with gain. The only line stage with gain I would consider is a DIY piece based on 12B4 tubes.
The I/P impedance is 200 K. :> ) If you use short, low capacitance, cabling to the amp, a passive control center built around 25 K. controls will be fine.
Eli D.
It doesn't look too difficult (see link), as long as you don't want tone controls.
Dave
The linked design will work with lots of power amps.
FWIW, my version of a 12B4 line stage will drive the fugly IHF "standard" load of 10 Kohms. You can use any well filtered and well regulated B+ PSU, instead of the pseudo choke I/P, pseudo dual mono, setup shown. If the PSU shown is of interest, I can provide info. about suitable magnetics.
Eli D.
I am not ready for a preamp build yet, but as vintage gear get more scarce and prohibitively priced. DIY will be the best option for many of us. I am currently researching tube power amp build, which I will be posting about on the tube DIY asylum in the future. I have read many useful post by you and others in the archives. When I build a tube preamp, the 12B4 is the best option I see, especially considering that the tube is readily available.
Dave
Hey Dave why don't you look into Bob Latino's kits over at tubes4hifi.
I have looked at his stuff. I like his 6SN7 version of the Stereo 70.
Dave
I still have my old Forplay preamp and its gonna stay in the family those are real keepers.
Bottlehead makes some interesting stuff. If I was willing (or $$$) to properly match (redo) a speaker system I really would like to try their 300b kit.
And what about that 300b prebee preamp Sweet!!!!!!
Yes Sir, I would like your source for that iron, Eli...
Email address
Clean time stream@
Yahoo.c o m
TIA
The Mind has No Firewall~ U.S. Army War College.
I do "hen scratch" schematics. The nice drawing was made by "Buzz", from a narrative I provided. The values shown reflect parts that were on his "pile".
For B+, a Triad N-68X is quite suitable. For heater power, a Triad VPL24-400 is suitable. A Hammond 193J (10 H./200 mA.) should be plenty "meaty" for the 1st inductor. Use a 10 Kohm/5 W. bleeder resistor. A pair of Hammond 155Hs takes care of the 2nd inductor requirement. Allied is probably the place to go for a full magnetics bundle.
Since the drawing was made, additional high PIV Schottky diodes have entered the marketplace. My current recommendation for the 4 diode bridge is Cree's C3D02060F (2 A./600 PIV).
The 0.47 muF. cap. in the "fudge factor" position is just a starting point. That value has to be tweaked to bring the rail voltage close to 125 V., but no higher. Stay under 1 muF., no matter what. The bigger the "fudge factor" part, the poorer the PSU's regulation.
The quality of the anode CCS used depends on the nature of the downstream equipment being fed. If the nasty 10 Kohm IHF load is being fed, use a superior, cascode, construct. OTOH, if a benign load of 50 Kohms or greater is being driven, a 10M45S is sufficient. Keep the 12B4 working into a net load of at least 3X R P and linearity will be maintained.
Eli D.
Thanks for the reply Eli. What you are saying is to set the amp gain at max and use a passive "25K" to set volume?
I think I have a pair of 25K Chinese attinuators with Dale resisters someplace in the basement!
Our Oppo DVD player has a setting that the volume can be used as well but I want it to be easy for our children to use. On the other hand maybe they can teach me how to use it.
"What you are saying is to set the amp gain at max and use a passive "25K" to set volume?"Correct! Attenuate the I/P signals with the amp's controls, only if you get a "hair trigger" situation.
Since you have a pair of 25 K stepped attenuators on hand, all you need is an aluminum enclosure, some RCA jacks, and a Lorlin brand non-shorting rotary source selector switch from Mouser. Eliminate a set of mechanical connections by making the cables to the power amp "captive". Make 3 wire braided cables yourself, from silver plated copper wire. Keep the cables short. Holding O/P cable capacitance to the absolute minimum is essential, when working with passive control centers. That's why you use braided, not shielded, interconnects.
Eli D.
Edits: 05/02/15
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