Double Bass Winter Project

[robgrow]

I had a derelict Polytone Mini Brute III from the early 80's laying around, with a damaged power amp and speaker, that I planned to use for something someday. That day came this winter, which just seemed to drag on forever. I got a neo Celestion BN15-400S speaker and an ICEpower 50ASX2 class-D power amp module to replace the old, damaged parts and kept the existing Polytone preamp, which I modified slightly. Not only are the Celestion neo speaker and ICEpower amplifier big upgrades to the original components, but they are really light too. So now I have a good sounding, very useful and reasonably light weight combo amp with respectable power (about 200 watts) for an investment of just over $200, and it was a fun project also.

 

[DoubleMIDI]

I would be interested how you build it together. Also what changes you made to the preamp. No need to hurry up, but it might be helpful for those who want to revive a dead Polytone, since the old power amp has a strange chip that is hard to impossible to get now.

My old Polytone Mini-Brute II still works (with the old power amp), but has a constant buzz that I would like to get rid of. Disassembled it wanting to adjust one pot carefully, but never had the time to do it. I even have ordered some replacement caps, trying to do a recap if the adjustment doesn't help.

 

[MrSidecar]

Rob,

some questions here, as I was once pondering something in that direction as well:

-Where did you get the module? I suppose you tore apart a time rumble 200 head or something? B&O don't sell single units to my knowledge.

-are you using the ICEpower in BTL-mode (bridged) to get the power rating you mentioned? If so, how did you approach the requirement for feeding the amp one signal as is and one with 180 degrees phase shift? Did you build a tiny opAmp-based inverter, or are you using discrete components?

-How do you power the preamp- do you regulate the -/+ 24 v aux supply off the ICEpower?

Thanks in advance.
best
sidecar

 

[robgrow]

Hopefully I can answer some of your questions...

I replaced the power amp in my Polytone because it had a burnt-out power transformer, and I couldn't see any point in replacing it with a new one.

ICEpower modules are available in small quantities from various sources on the internet. I bought the 50ASX2 on ebay for $99 USD (about 92 Euros). It is available in two versions: SE for dual channel use, and BTL for single channel use. I used the BTL version.

The Polytone power transformer has two secondary windings: a higher voltage one for the power amp and a lower voltage winding for the preamp. The leads for the lower voltage secondary winding go directly to the preamp, which has its own small onboard power supply complete with IC voltage regulators. I connected the auxiliary +/- 24VDC from the 50ASx2 to the preamp's power supply inputs, where it is regulated down to +/- 15VDC.

I replaced the 4136 op amp in the preamp with a BIFET part, which required some creative rewiring, since it has different pinouts. I think I changed some component values too. None of this was absolutely necessary though.

The original Polytone speaker had a big hole in it, so I decided to replace it with a higher-quality neo driver, which I also bought on ebay (for $112 I think).

 

[MrSidecar]

Hopefully I can answer some of your questions...

I replaced the power amp in my Polytone because it had a burnt-out power transformer, and I couldn't see any point in replacing it with a new one.

ICEpower modules are available in small quantities from various sources on the internet. I bought the 50ASX2 on ebay for $99 USD (about 92 Euros). It is available in two versions: SE for dual channel use, and BTL for single channel use. I used the BTL version.

The Polytone power transformer has two secondary windings: a higher voltage one for the power amp and a lower voltage winding for the preamp. The leads for the lower voltage secondary winding go directly to the preamp, which has its own small onboard power supply complete with IC voltage regulators. I connected the auxiliary +/- 24VDC from the 50ASx2 to the preamp's power supply inputs, where it is regulated down to +/- 15VDC.

I replaced the 4136 op amp in the preamp with a BIFET part, which required some creative rewiring, since it has different pinouts. I think I changed some component values too. None of this was absolutely necessary though.

The original Polytone speaker had a big hole in it, so I decided to replace it with a higher-quality neo driver, which I also bought on ebay (for $112 I think).

Thank you. That sounds as if there is a difference between the 50ASX2 and the 125ASX2, the latter being suited for both dual-channel and BTL use. If there's a dedicated BTL version of the 50ASX2, that may eliminate the need for turning the phase on one signal.

Sounds like a cool project, good job!

best
sidecar

 

[robgrow]

Since this whole project was something of an experiment, one thing I did was use a small line input transformer at the front of the ICEpower module. I had the transformer* laying around, so I decided to give it a try. I'm a fan of the old Walter Woods MI 100 amps, and the saturation characteristics of this transformer does a pretty good job of simulating the sound of the class A preamp on the MI 100. Also the transformer is a bandpass filter and keeps out everything above and below the audio band, while being reasonably flat within band. An IC op-amp-based balanced line driver worked well too, but I like the sound of the transformer, which adds a little warmth and focus.

*Crimson Audio 10K bridging input transformer with 1:1 turns ratio