Classic Audio- the RTS Systems HPM-41 mixer

As much fun as all the latest and greatest software-based audio technology is, I still like old analog circuitry. It has it’s quirks, of course, but there are lots of cases where high-quality analog still sounds outstanding… as good as, if not better than, digital.

The front panel of the HPM-41 mixer by RTS Systems

The front panel of the HPM-41 mixer by RTS Systems

One of the mixers in my collection is the RTS Systems HPM-41, which is a 4-input mono mixer. Background info on this unit has been very difficult to find. RTS Systems abandoned the mixer business years ago to concentrate on intercom systems for television and broadcast stations. It looks like these were built to compete with the ubiquitous Shure M67 4-channel mixers in use in  just about every radio and TV station in the country. The Shures are built like tanks… dependability is critical in broadcast… but the RTS mixer has some big advantages over the Shure. Some are apparent from the front, and others you can see once you open the unit up.

The rear panel of the HPM 41

The rear panel of the HPM 41

Each channel has a two-position pad to drop the input signal 15 or 30 dB. There’s also a 75Hz and 150Hz high-pass filter, and a real bonus, individual limiters on each channel. On the back, each channel can be phase inverted, and channel 1 and 2 has phantom power, which can be switched between 12v A-B powering or 48v phantom. (I like this arrangement better than my brand-new and very expensive Shure FP33 mixer. To change the phantom powering, you have to open the unit up and poke around the battery compartment. It’s tight, your fingers are constantly touching the circuit boards, and it’s easy to forget the settings.)

All the switches on the HPM-41 are recessed toggle switches, which gives the whole thing a higher-quality feel.

This mixer is really heavy. This is due to the large mains transformer inside the unit. While it has a provision for battery power, it’s really designed for stationary operation rather than fieldwork in a bag.

Inside the HPM-41. The large rectangle on the left is the RF shield covering the AC mains transformer.

Inside the HPM-41. The large rectangle on the left is the RF shield covering the AC mains transformer.

Once you open the unit up, the differences between this and a Shure M67 become immediately apparent. The pots are Bourns sealed type, probably conductive plastic. These give better performance than the usual carbon type. Four big electrolytic capacitors are also immediately noticed. The 25-volt, 3300 uF monsters will be hard to replace since they are axial-lead type, and this type of lead is not  normally stocked in better grades. Electrolytic capacitors can degrade over time– especially through lack of use– and it’s usual to replace them in vintage equipment. But capacitors that are larger than their minimum requirements do last longer. I’ve even heard it suggested that with caps, more is always better. (but within reason, of course.)

Beyer input transformers

Beyer input transformers

ICs in this unit are 5532s and TLO84s. While there are some other IC’s that might yield higher performance (such as the Burr-Brown 2134), the general consensus seems to be that 5532’s are pretty good chips. Also noted, but not usually seen, are four Beyer Dynamic German-made audio input transformers. Some good info on these and other circuits can be seen on the kubarth site.

The RTS Systems HPM-41 is a comparatively rare mixer. My guess is that these were very expensive units judging from the components, especially when compared to the Shure utility mixers from the same period. I don’t use mine very much… the weight and power requirements mean that I can’t use it in a bag, running around under battery power, which is 95% of my business. And it’s usefulness is further limited by being a mono mixer. But I expect to find a good use for it as a preamp or base for modification. Several ideas come to mind, including placing the heavy power transformer into a separate case, changing out the power plug to a 4-pin xlr, and adding direct outputs. If you happen to find one cheap, I’d go ahead and buy it, or let me know about it. Also, if anyone has any documentation on these, I’d very much appreciate a copy and I’ll add it to these pages.

11 responses to “Classic Audio- the RTS Systems HPM-41 mixer

  1. I designed that mixer at RTS Burbank many years ago.

    Not sure about documentation, there might be some scraps somewhere. 805-388-8848

    • bgilbertsound

      Hi Bill:
      Thanks VERY much for your comment… needless to say, I’m really impressed by your work… wish I had your mental guns!
      I have two of these mixers. One is in pretty good shape, I’ll likely leave it as-is, or repair/restore as needed. The other one is pretty much beat. This one will be my experimental platform, when I get some time. One of the things I want to try is tap the signal at some point to try and get direct outputs for each channel, turning my beater mixer into a 4-channel preamp of sorts… do you think it could happen?

  2. Just picked up a minty one of these and am very excited to put it through the paces as part of my home recording setup.

    Thank you for posting this info, as it’s pretty much the only info on the unit that I could find anywhere.

    Also interested in a mod for direct outs… did you get anywhere with that? Did you ever find any more documentation? Thanks!

    • bgilbertsound

      Hi Matt: I never did find any other info on these things. I have opened one up to find a burned resistor that was buried pretty deep. I’ll post a photo on the site. I never did try to get a direct out mod done… just one of those projects that I never could find the time to work on. The output transistor is an issue, since the best implementation would require four more… no room in the box. Removing the power transformer to a second floor case might help, though

  3. I was guessing that being summed to mono by design, it would need 4x more output transformers for balanced outputs, and I doubt one would want to go to the trouble of modifying it for unbalanced.

    It would seem to me that maybe the best course of action (presuming you were wanting to use it in a more stationary studio setting) would be to re-house it in a 2U rackmount chassis that would offer room for extra transformers and space to stand off the power transformer a bit… similar to the route people take racking channel strips from old consoles for individual use.

    I’ll be doing my own listening, but I hardly have golden ears, so I’ll ask you: how does this unit sound? In your opinion, is it a high enough quality unit to warrant the mods? I know just enough about old broadcast gear to be dangerous… i.e. I know some of it was really spectacular, and some was only so-so.

    • bgilbertsound

      Matt: I think they sound pretty good, but I haven’t done any critical listening comparisons. It will need to be in good shape to be useful… some of these are beat to hell. They’re pretty old now, and would certainly benefit from a recap, which would be expensive because of the size of the caps. I don’t really use mine that much, since I like the preamps in my Soundcraft. But the transformers add some second-order harmonics that will give you a “sound” that you might like on certain applications.

      Old broadcast gear can be lots of fun, since it was often made really well, though that isn’t universal. Open up an old Shure mixer and you’ll see what I mean (and those things weren’t cheap, either!) ADM had some great gear (I used a 1600 console for years to switch a news show), but my very favorite was a Harrison analog broadcast board. Unfortunately, I can’t afford anything by either of them… except for Harrison’s Mixbus, which I recommend highly!

  4. Stan Hubler June 29, 2013 8:30pm
    Hi Brian, Doug Leighton, Stan Hubler and Rick Karwoski cofounded RTS in 1975 at Doug’s house in Studio City California.
    Along about 1980 or so as I kind of recall, Bill Eisenberg and Doug Leighton designed the HPM-41 to compete with the Shure unit. It was kind of a side project at RTS and I don’t recall it going through the regular engineering channels. I normally wrote the manuals for almost every product but I think the HPM-41 manual was a separate project. I just looked through all the manuals I have and didn’t see the HPM-41. I am presently writing a history of RTS and just got to the page for HPM-41. I searched on the internet and found your excellent website. I’ll check with Doug and see if he has a manual I can copy, in which case I’ll send it along. As you may know Bill Eisenberg passed away during this last year. He was such a brilliant audio engineer.
    Best regards, Stan Hubler (Former Vice-President/Director of Engineering at RTS Systems).

    • bgilbertsound

      Hi Stan:

      Thanks very much for your note. As you can tell, I’m a big fan of your work… I still have an RTS mixer and it still works, though I don’t really use it much and I can’t bring myself to modify it (for individual outputs from each channel, for example) as I would quite probably just screw it up. I may yet try it if I can find an old beater (and a bunch of output transformers-$$), as they occasionally appear on eBay at a realistic price. But they’re beautifully designed and built, and they sound great… they must have been a great deal more expensive than M67s. Still, I can’t understand why more stations didn’t use them. I’ve taken apart a few Shure mixers, and there is really no comparison.

      I’d be very interested in reading your history when you get it done.

      Thanks again

  5. Wonderful mixer. Mine had a resistor and diode burn up so I rally could use a manual or at least a parts list. These two components are totally black and I can’t see any values.

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