BOGEN MPR-6 Mixer with Spring Reverb Repair and Modification

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Mike Zee
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BOGEN MPR-6 Mixer with Spring Reverb Repair and Modification

Post by Mike Zee » Fri Mar 07, 2008 5:57 am

Hi , Guys,
this is about BOGEN MPR-6 unit with spring reverb, which you may find somewhere or on eBay and maybe for cheap, so if you see one - get IT 8-)
I've put together page for this project with all the details and photos there.
Here is the link:
http://www.mzentertainment.com/studio_w ... mpr_6.html
This is a solid state unit (all transistors, and it's pretty simple electronically).
Originally it has 6 mic input channels with 6th channel AUX input, each channel has "send to reverb" switch OFF/ON, it has reverb-in-the-mix Intensity level and output level control.
Each channel has Bass/Treble.
So it's pretty cool as is, it's good for mics mixing(submixing) and adding spring reverb, also you can just plug guitar into mic input.
*********
The modifications in the project were:
Adding Send-To-Reverb Level Potentiometer Control on each channel and adding Line-Level input Jack on all channels (1-5), so you can use it for dubbing more "effectively"
I've made various detailed photos inside and all the details on how to modify it.
I have not done much with it in dubbing applications yet, just finished up all the work and did some general testing with guitar etc...

Let me know if any questions or comments.
:D

/respects
/mz

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JahNice
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Beautiful....

Post by JahNice » Fri Mar 07, 2008 3:12 pm

Hi Mike! Now this looks like a very nice Dub-Toy to me. Its always fun to watch the workshop projects on your website, but this old spring reverb mixer project is extra exiting to some Dubhead like me.. Very nice & clean working. I dont know much about electronics, so it would had been nice if we had some sound-samples from befor the modifications you made. Just to see what the differences in the Sound would be.
Keep up the good work..

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Post by Dub Attack Force » Fri Mar 07, 2008 3:53 pm

Hi Mike

Nice find! That's a cool-looking unit. I trust that this is an excellent unit for dub purposes. Can you tell us how many springs this device uses, and the dimensions of the springs? Also, do the 'bass'/'treble' filters come before or after the springs? Is there an actual amplification buffer at the end of the circuit, or does it use unity gain buffer(s) between filter stages?

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Post by Mike Zee » Fri Mar 07, 2008 4:18 pm

Hello, JahNice
thanks a lot for checking it out and replying :-)
*******
As for "sound before (or after) modification", there is no really difference in terms of how the mixer or reverb/mix "sounds", as I have not change anything in the audio path.
I've just added control and inputs. So it's the same mixer but with more features :grin:
The potentiometers on each channel that I've added only provide control over how much signal you want to send to reverb from each individual channel. Before that, all you could do was to turn reverb ON or turn OFF ("send to reverb" that is) on the chanel by flipping the switch (so it's like having send-to-effect button (aux send) but no "send knob" on a mixer).
And another modification was to add Line-Level inputs. Before you only have mic level input, which means that the signal goes through a mic preamplifier, and if you plug a line-level signal into it - you get too high signal level and it gets overloaded and distorted.
So the line level inputs are there "bypassing" the mic preamp stage and go right to channel's level (volume) control (the fader) and then to chanel amp stage and then to Bass/Treble passive eq (tone control).
So this second mod is kind of important if you want to use this unit as spring reverb effect when dubbing (where you send line-level signal from your dubbing mixer to Spring Reverb unit). And the coolest part of MPR-6 is that it is a spring reverb, but you can send up to six different parts (instruments, tracks, channels) while dubbing, and have a lots of control over the situation and dub on the fly LIKE YOU MEAN IT :grin: :ganjabounce:

Also, nice thing about this unit is that it can be used simply as spring reverb effect or as mic mixer, this would be appreciated by dubbers who record acoustic instruments, drums and what ever with microphones.
Also at the same time it is a siple yet pretty good preamp with reverb for a guitar player. You can simply plug electric guitar into mic input and record your guitar to what ever recorder or computer from the output of MPR-6 and you can add spring reverb to guitar while recording, which is always cool thing.
***************

So for all these reasons I've decided to take time to do all this detailed work to demonstrate what this unit is about.
Just wanted to share this with Dub-Men 8-)

I've seen these mixers out on eBay, not often though, but sometimes, and they sell rather cheap, maybe most people don't see them as much useful or something.

I think there are also some older TAPCO mixers with spring reverb in it which may be similar, but I don't know for sure (and couple or few others).
***********

God Bless
and keep on Dubbin'

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Mike Zee
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Post by Mike Zee » Fri Mar 07, 2008 4:37 pm

Dub Attack Force wrote:Hi Mike

Nice find! That's a cool-looking unit. I trust that this is an excellent unit for dub purposes. Can you tell us how many springs this device uses, and the dimensions of the springs? Also, do the 'bass'/'treble' filters come before or after the springs? Is there an actual amplification buffer at the end of the circuit, or does it use unity gain buffer(s) between filter stages?
Dub Attack Force,
I do not have printed schematics for the unit, but the electronics there are pretty simply and I'd say "minimal"
I have detailed photos and description of the boards and if you know the basics it's all pretty much self-explaining, please see this photos image, here's direct link:
http://www.mzentertainment.com/pics/bog ... ations.jpg

here's image with photos of inside the tank:
http://www.mzentertainment.com/pics/bog ... oject3.jpg
the tank length in somewhat 14-15 inch (?) I did not measure exactly, it's a pretty regular tank, two springs.

The EQ is passive, it's after channel amp and before mix (summing amp).
Send to Reverb is "taken" after EQ, first all six "send to reverb" signals are mixed, then amplified (it's two transistor circuit). I may be incorrect, but it may be current amlifier (the second transistor - PNP), as the signal from the collector of the second transistor goes to the tank.
The recover amp is a one NPN transistor amp, then it goes to "reverb intensity pot" and to "mix"-amp and then after channels signals and "from reverb" signal are summed - it goes to final "output" amp.
I hope, my "description" makes sense. See the pictures :D

Please, let me know if you may have specific questions about electronics, I may be able to check the details.

/respects

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Post by Dub Attack Force » Fri Mar 07, 2008 4:51 pm

Great photos! I wish I had that unit... It looks like a high quality piece of kit.

Yes Mike that explains it perfectly. Just a couple more questions if you don't mind: do you use the tone controls and how do they affect your music, how many orders of passive filter are there for bass and treble stages, and what are the frequency cutoffs for bass and treble? It would be cool to know this information, to get a feel for the old sound.

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Post by Mike Zee » Fri Mar 07, 2008 7:00 pm

Dub Attack Force wrote:Great photos! I wish I had that unit... It looks like a high quality piece of kit.

Yes Mike that explains it perfectly. Just a couple more questions if you don't mind: do you use the tone controls and how do they affect your music, how many orders of passive filter are there for bass and treble stages, and what are the frequency cutoffs for bass and treble? It would be cool to know this information, to get a feel for the old sound.
to be honest I can't answer your questin :sad:
I just did not care to know
heh heh :think:
More to that, I really did not even care to take close look what exactly components values and circuit is there for bass/treble (you can try to figure it by looking at the photos, also the cool part of it is that the connections of the PCB are actually drawn as white doted lines, so you don't need to flip and look at the bottom of the board),.
I only can tell you that it seems that there's nothing really special about it.
The unit is designed (as I understand) simply for some microphones-on-stage situation, maybe even not for music really, also I have no idea. So, it's nice to cut some highs to control "mic feedback" and cut some lows to make "voices" sound clear. That's what I think what this thing was designed for. Maybe not. How do I know, I m just guessing.
To me personally it was not that important even to have any eq there, but since it's there, it's just even better.
As for dubbing application, you know, you always can (and most likely will) control tone before and after spring either on you 'main' dubbing mixer or use dedicated EQ unit(s), that's if you really are after some special and demanding sound, you can't just rely on (nor expect much from) a simple bass/treble filter network, know what I mean?

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Post by Dub Attack Force » Fri Mar 07, 2008 7:50 pm

No worries Mike - thanks for all the information you have supplied.

I was curious about the filter characteristics because it seems to me that filters are extremely integral to a particular sound. Also, I have a 2-spring reverb unit (without filter stages) and since I usually put a guitar EQ in the chain after it I thought it might be interesting to know the specifics of what the old-timers thought was right for spring reverb EQ. For example the old-school 'bass' boost level on hifis was 100hz, which is quite a lot higher frequency than today's preamps. So from this we know that what was considered aesthetically pleasing is different from what we have now.

Unfortunately from the picture it is not possible to know the values of the capacitors so not possible to work out the frequencies from there. Thanks for the great info and pics of this unit. Hope you put it to some good use. :D

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interruptor
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Post by interruptor » Sat Mar 08, 2008 5:48 pm

Indeed Mike.. this thing looks like the absolute uber dub weapon! :twisted: And after your restoration job it's sparkling like a piece of jewellery. A dubbist's bling bling, Ha!

The spring tank looks exactly the same as the one on my spacexpander. Also it has that crucial wobble when fed with percussive sounds (can be heard in one of your sound clips where the guitar or cable generated some pops/clicks).

As this box has only one effect send (to the spring) you wouldn't use this as your main dub mixer. But it could be used as a spring reverb unit receiving it's signal from one of the auxes on the main dub mixing board. For this purpose it would be best to get the pure reverb signal (100% wet) out of your box not a mixture of the input signal and the reverb. Is this possible?

cheers
Daniel

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Mike Zee
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Post by Mike Zee » Thu Mar 13, 2008 4:55 pm

Hi, Daniel! How are you? long time no blah :-)
Sorry I've missed your post few days ago :-(
**********
Good point on "reverb out". To answer your question - technically as the mixer designed you can't get "wet only" reverb out. It can be added as modification, I did not think about it.
I've made a simplified block diagram of the unit, here's the link:
http://www.mzentertainment.com/pics/bog ... iagram.gif
********
Also, as it is, when you turn intensity all the way up - the output sound gets very wet :D . Also, see how the intensity and output pots are connected. I am not sure why it is done this way (the signal goes to the wiper and is taken from the "top"). Well, that's the way it is done there.

And, yes, I would not think about using this mixer as "main dubbing board". You can see this device (in a sense) as a spring reverb effect unit with multiple inputs each with its own level and tone control - and this is a real cool situation for dubbing (especially on the fly 8-) ), well, assuming your main board has a lot of channes, direct out and sends :grin:

I'd say, also, imo - the real cool thing about it, that for a dubster with no much cash in the pocket this can be a real great deal, as you get a lot of useful features and it is also a "guitar preamp" and a mic preamp, you can use it as: simply plug guitar in (or mic - for vocals, melodica or some percussions etc) and send the output to couputer's audio-card or what ever your recorder may be. Not a fancy thing, but it sounds rather cool and "vintage too !!!!" :-D

/God Bless

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