Thursday, March 25, 2010

Resurrecting the McIntosh MC-30

The Mighty McIntosh

My boss Jack has had a pair of old dusty and slightly pitted MC-30's laying in his office for several years. The story was that they belonged to his father who purchase them new sometime in the golden age of Hi-Fi, and they had been languishing at the home of one of his other sons for years. Jack came across them and rescued the pair thinking that someday he would put them to use driving a pair of Renkus-Heinz commercial 16ohm speakers we've had in our warehouse, also collecting dust. Well, he finally asked me if I could do something with them.. duh!

I got them home (heavy buggers!) and got one up on the bench, opened it up and found that some repairman from the distant past had done some meatball surgery that had to be corrected. One of the amps power supply section was really messed up by someone - the rectifier feeds a 30uf cap first, then a 150 ohm power resistor to an 80uf section forming a CRC filter. But what I found was actually a 10K power resistor feeding the second stage filter (which is B+ for the output stage). I don't see how this could have even worked! Plus, there was a 12au7 instead of a 12ax7 in the driver cathode follower stage. Also, the power cord had long since disintegrated and there were some burnt wires (why did they use 22 gauge wires from the rectifier to the filter cap??), but overall things looked pretty good.

First things first, get rid of all the 45 year old electrolytics! Except for the main filter caps, which measured good with the cap meter (all 3 sections), there were dried up bias supply caps and input stage cathode bypasses that had to go. Also, the paper/wax coupling caps were suspect; I've heard that they tend to get leaky- though some audiophiles like the way they sound. I don't. I like detailed music reproduction, thank you. These caps would be good for a guitar amp where you're going for a colored sound, not Hi-Fi (now days there are very expensive 'boutique' paper in oil capacitors available, I'm not talking about these). Most of the other coupling caps are some sort of plastic encapsulated film, not sure about them. I left them in for now.

Since Jack is not an audiophile I didn't use any exotic parts; rather than an 'upgrade' this project is more of a resurrection. I used what I had on hand. I replaced the caps listed above with standard modern electrolytics, replaced the output stage coupling caps with poly film (.47uf @ 630V) and installed grounded power cords (I cut the IEC ends off of some stock power cords that come with AV equipment). I also put in some 330uf @ 450V Panasonic snap-in caps that I've had laying around since the '80's (these were in the first iteration of the ESA 66-100 which have long since been scrapped for parts). These I paralleled with the 80uf main B+ caps, and bypassed them with some .1uf 450V poly's.

I do see alot of room for improvement, though. Like replace all the old coupling caps with decent film caps like Solen or better, do more power supply bypassing for the input and inverter stages and get rid of all the unnecessary input wiring and hardware. The input RCA's could use an upgrade, and a heavier barrier strip so you can connect some 'real' speaker cables would be nice.

The design of the amp is interesting. There's the obvious 'Unity Coupled' output transformers which have a cathode winding from one tube in phase with the plate winding of the other tube. There is the 'bootstrapping' of the driver; the B+ for the driver load resistor is derived from the plate winding of the same phase tube giving a positive feedback to the driver tube while delivering negative feedback to the output tube. And there's the 12ax7 cathode follower between the driver and output stages, direct coupled to the input grids and therefore having the -45V bias voltage on their cathodes. This tube is also bootstrapped; the positive feedback is necessary to derive the very high AC drive voltage required for the output stage (remember the cathodes are in the transformer winding causing negative feedback, lowering the overall gain of the stage). The input stage is pretty standard and uses 1/2 of a 12ax7 direct coupled to a 12au7 configured as a cathode coupled phase inverter. The McIntosh design was way ahead of its time and the performance is reported to be excellent. All in all, alot of circuitry for 30 watts!

Ok, so after all the circuit work, I cleaned up the chassis and tubes, plugged them into AC, and checked out the voltages. One amp measured spot-on, the other had some funkyness in the output stage. One output tube was in cutoff and it's bias voltage measured -220v! Some visual inspection revealed that I had inadvertently cut a 220K cathode resistor for one of the followers, which supplies bias voltage to the grid of the 6L6. Repairing this error fixed it right up!

Alright! I'm too impatient to put these things on the scope and do the requisite battery of tests, I just want to plug them in and play some tunes! They sound pretty fine, I must say. They're a little dark and the bass is not as tight as I'm used to (the 66-100's are highly resolving and have excellent, tight bass). But overall, they have that tube magic. I bet new output tubes and performing the rest of the upgrades as listed above would bring them into better focus, but my boss just wants them to work and I think that they'll do just fine with the high-sensitivity horn loaded Renkus-Heinz speakers he's going to drive for his office system.

I promise I'll get these things hooked up to the scope and see what the power, frequency response and square wave look like, but for now I'll just enjoy them before I have to give them back!

UPDATE: 3/28/10

Just put these babys on the test bench. Amplifiers terminated into 8 ohm resistive dummy loads.

Square wave response is good for 1Khz, 10Khz is a bit rounded off on the leading edges, suggesting a bit of high-freq. rolloff.

Freq. Response is flat from about 30hz out to about 15Khz, and there's a bit of droop on either extreme, as viewed on an o-scope.

Power is 45 watts before visible clipping on the amp with the better output tubes. The one w/ the weaker set puts out about 25 watts before clip. Think I'll recommend new tubes to Jack.

No distortion measurements made at this time.

I had fun restoring these amplifiers. They are very well made, well laid out and easy to work on. I'm sure Jack will enjoy listening to them as much as I have!

Tuesday, March 23, 2010

Home-brew high end speaker cables

Becoming interested in a few DIY projects seen online I decided to make my own speaker cables to replace some home-made jobbies that I'd been using for about 10 years..

Since I work in the commercial A/V field I have collected a ton of surpluse cable and wire over the years. Having an abundance of CAT5E cable on hand it was a cinch to choose one of the projects that feature use of that sort of cable. The final design of the cable is a distillation of several articles that covered the making of your own speaker cable.

Needing approximatley 15' runs I played out eight 18' strands of CAT5E cable. I then made four twisted pairs out of the 8 strands, twisting counter-clockwise. I then took two twisted pairs and combined them, twisting in clockwise direction. This made two rather thick braids of CAT5E about 16' long, of which I secured the ends with electrical tape.

Next I cut 4 6" long pieces of heavy 1" heat shrink and slid them down the cable ends. Then stripped back about 6" of each of the four strands at each end. Next I un-twisted every pair, sorted them out (stripes and solids) and combined all the solids together and the stripes together. I then twisted each bundle and wrapped with teflon plumbers tape. Next I pushed 3" of red heat-shrink down the "solids" bundle and 3" of black heat-shrink down the "stripes" bundle. Next, shrink the black and red wraps on the bundle ends, pull the 1" x 6" heavy shrink up the cable to cover the transition point where the bundles are seperated out of their CAT5 strands and shrink. I then put some hot glue into this cover between the bundles, heated and pinched together with big needle nose pliers to clamp the shrink together and make "pants" over the transition point and the bundles.

Next I stripped each individual strand of the wire bundles for pos. and neg., trimmed to length and crimped on heavy fork connectors, then soldered with silver content solder. Heat shrink was applied over the connectors for a finished look.

Upon first listen they sounded terrible. But after about 1/2 hour of listening they started to break-in and sound better. Now with about 6 hours on them they sound wonderful! Tighter bass, more texture and detail in the lower registers, better integration of the whole sonic picture and a more even presentation of the frequency range. More detail is heard, more low-level stuff like reverb tails, backing vocals I'd never noticed before, etc.

Since CAT5E cable is rated for 350Mhz it should work fine as an audio cable. Even better is that the positive and negative conductors are all seperated and twisted together in individual pairs, and each individual CAT5 strand is twisted with another, and so on. This results in very low inductance (but rather high capacitance, though that doesn't matter as much as inductance for speaker cable) and low resistance. Also this design eliminated the smearing of signal and skin-effect losses associated with multi-stranded cable because each strand is individually insulated, much like Litz wire.

Anyway, DIY is awesome, and cheap!

Friday, March 5, 2010

Power Conditioners and hype..

In my day job I am a service technician for a medium sized regional A/V contractor. Most of my work involves troubleshooting audio systems and optimizing performance. I have the opportunity to work with some very nice equipment in beautiful environments, like some of the local church sanctuaries around Seattle. One such system involves a church that is used as a venue for performing and recording sacred music, especially involving period correct interpretations of the music score with appropriate instrumentation. The acoustics of this church are perfect for the exquisite pipe organ and musicians; not so much for the spoken word (the two are often mutually exclusive and our job is to try to integrate sound reinforcement into these challenging acoustic environments).

Last year we erected a 25' scaffold in order to install 4 hanging mics from the very tip of the ceiling- high-quality Neumann condensers, two at about center stage and two out in front of the stage and about 12' off the floor. The cabling was run to a mixing console in the rear of the sanctuary and then fed to an Alesis Masterlink hard-disk recorder. The sound captured with this set-up is incredible, but the mics are so sensitive that they can pick up a whispered conversation at the back row!

The music director and the recordist / board operator had been reporting an intermittent noise coming through on the recordings. Intermittent problems are difficult to troubleshoot and the first time out I couldn't re-produce the problem. The second time out I was able to hear the noise through the monitoring headphones. It was a nasty intermittent buzz that varied in intensity and character. We swapped in another console and the noise went away, so I determined that it must be something in the consoles PSU and sent it away for service.

They actually did find something wrong- the phantom power supply was bad and they replaced some components. But after installing the repaired console back into the system the buzz was still there. ??!! Then I noticed when we moved the power cables around behind the desk the intensity of the buzz would vary, and this led us down the path of changing out extension cords and eliminating some cheap plastic power strips. Also, uncoiling the power cables and laying them flat under the desk took care of some more of the noise, but it was still there to a degree which was clearly audible on the recordings.

I then recommended another option that I thought would work since it seemed due to dirty power. That was installing a high-quality series mode power conditioner in the system. The Surgex SA-82 fit the bill and was ordered. These units are expensive but they really do the best job of surge suppression, and unlike MOV based surge protectors they last forever and don't dump dangerous and noisy voltage to ground. The side benefit is that because they use a large series choke to do the surge suppression they filter out way more noise and nasties on the power line than do the tiny little RFI filters found in most professional surge suppressors.

Well, it worked like a champ. All of the noise, buzz, hum and grit was gone, and the sound has taken on a more relaxed and open quality. They really do work. Check out their website for more information on how they work:

The power grid is getting noisier all the time; everything has a switch-mode power supply these days, and those CFL light bulbs they're pushing on us throw out noise too. Plus the utilities can at times send big voltage spikes into your house when working on the lines, it's happened to me! Cheap insurance to protect your stuff.

So, the hype is that these units do an excellent job at protecting your equipment at the same time cleaning up your power from noise, and are much less expensive than some of the crazy audiophile type power conditioners out there.
I think I'll get one for myself!