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Old 17th December 2012, 09:49 PM   #1
vita is offline vita  Switzerland
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Default Sumo Andromeda predriver advice

Hi all,
Please, I need some advice on this amplifier, because after ten years of SET, I needed an amp capable of driving speakers at 2 ohms, after trying it, it is his job without problem, as the sound is pretty good, I wanted to restore it, as some components are not original.
This is where I need your skills, whether it's worth changing the Predriver, are the transistors Q3, Q4, Q5, Q6 on the diagram.
On my amp they are MPSA05, MPSA55K, they seem to be matched (small paint strokes).
I wanted to put the sa818y, sc1628y, they are expensive, but I can get them,
bandwidth is not the same, is that it can change the sound?, is that it is a mistake to try to change them?
Transistors for power on the left channel, there are ARC4700C, 4800B, for the right, MJ15024, mj15025.
There are some small differences sound between the two, is it better to put the same motorola, or the ST2121, ST5949?
I also have a question for Mr.llwhtt, please, is it I could get the procedure to adjust this amp?
Thank you,
Best Regards.
Stefano
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Old 18th December 2012, 04:10 AM   #2
llwhtt is offline llwhtt  United States
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The transistors you mention are the input differentials, the pre-drivers are Q11,12,13, and 14. I will look for my Andromeda service manual tonight, but I believe it's 14mv across the emitter resistors, R65-R72.

Can you get a schematic for an FM Acoustics FM300A amplifier? It's made in Switzerland and there is absolutely no info here in the US. I have a butchered one here and could use a schematic. Thanx.

Craig
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Old 18th December 2012, 05:14 PM   #3
llwhtt is offline llwhtt  United States
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I found my Andromeda I service manual and the bias adjustment is as follows:

1. With power off, face the PCB from foil side and remove fuse on left, closest to the edge of PCB.

2. Connect DC ammeter across fuse clips. Turn power on and observe meter for 60ma + or - 5ma while heat sink is cold. Current will drift up as heat sink temp. rises. No load, no signal.

3. Adjust R73 if not in spec.

4. Turn power off, replace fuse.

5. Repeat for right side of PCB, remove fuse, connect ammeter, turn power on and check for 60ma, adjust R74 for 60ma if needed.

6. repeat for other channel.
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Old 18th December 2012, 11:50 PM   #4
vita is offline vita  Switzerland
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Hello,
thank you for your reply,
I was writing my text, when you had already answered, I put it as is:


Unfortunately, I do not know anyone with this type of amp, their website does not say much, apparently, FM, wants to maintain a certain mystery, only few elected through a strict selection can afford the object desire, or something in this genre there.
The only time I saw a semblance schema, as well as several photographs of printed circuits and components
it was the site of a repairman Japanese
by the translation of the text, it has Interogation points on some transistor, probably custom built without reference.
It was one of the first FM I can not remember the model, there was a cooling tunnel horizontally arranged inside the chassis.

For the Sumo, I want to explain some symptoms,
when I turn it on, it makes a little plop in speakers,
then by vibrating against the transformer, it is a cyclical noise increases, and then it almost disappears, but I hear a grinding noise in the speakers, is that the diode bridge is tired, or capacitors, or improper adjustment.
Channel with Motorola, heat a little more than the channel with the ARC.
Anyway, I'll change all capacitors including ceramic.
For the imput differential, Q3-Q4-Q5-Q6 (thank you for correcting my mistake),I would like to change by the original or replacement, according to Japanese datasheet (sa1145-sc2705), if there is an effect on the sound, otherwise if you're telling me that MPSA05 and 55 are very good, because it does no difference in this case I do not exchange.
There are also Q15-Q17, Q16-Q18, 3 are 2sb595y (hfe120-240), and 1 2sb595o (hfe70-140), the complementary transistors 2sd525y were replaced by TIP31C.
You surely understand, I'm not electronics, but I think that all these components, which are not original (unless there have been several versions of this amplifier) can affect the overall balance of the amplifier.
I have several multimeters, but no oscilloscope,
Is an adjustment can be made into 2 ohms with multimeter?

Best Regards
Stefano
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Old 19th December 2012, 12:13 AM   #5
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the procedure is simpler than I thought,
Tomorrow, I'm going to measure the current state, to see if the noise disappears,if they have something to do with the setting.

Best Regards
Stefano
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Old 19th December 2012, 02:46 AM   #6
llwhtt is offline llwhtt  United States
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Using a current meter is much easier than trying to get to the emitter resistors on that amp, I sold my two Andromedas almost 20 years ago so I forgot about the mechanics of that amp. If you have two DMMs you can set the bias on both sides of one channel at the same time.

Craig
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Old 19th December 2012, 03:14 AM   #7
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Stefano,
Thank you for posting the schematic for this amp. I have always liked the Sumo amps and it is to bad that the company went away. I wouldn't mind building one of these amplifiers myself. Don't know that I can but perhaps with the right help here on this site I could do it.

Steven
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Old 20th December 2012, 02:26 AM   #8
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Hello,
I just finished adjusting the two modules, as described by Mr.craig.
For the left channel, there was an imbalance of 22mA, ie: 91 and113mA,
for the right channel, it was not better, it also warmed more, as I described earlier: 34mA, 93 and127mA, the bias was set rather hot
I adjusted at about 62.5mA.
DC is +-46.5volt.
Then I wanted to see how it sounds, I would say that before the sound was pulsed
a little faster and a little more control at the bottom.
I have this amp for about three weeks,
it would take a long time, it was set like this, because the previous owner did not know that the power transistors were changed.
Provided it is well balanced, I could keep the bias a little hotter, or it creates a problem of cross between transistors?

Best Regards
Stefano
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Old 20th December 2012, 02:54 AM   #9
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Stefano,
Thank you for posting the schematic for this amp. I have always liked the Sumo amps and it is to bad that the company went away. I wouldn't mind building one of these amplifiers myself. Don't know that I can but perhaps with the right help here on this site I could do it.

Steven


Fortunately, the schematic is on the internet.
I do not know much about the topologies of the amp, I read that this amp to be placed on the market, was considered excellent,
at an affordable price, this is perhaps why it is not so much copied.

Stefano
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Old 20th December 2012, 03:17 AM   #10
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Stephano,
When I was showing my speakers at the CES show in Las Vegas one year Sumo loaned me amplifiers to use during the show. I liked the sound very much and thought that later I would purchase one of their amps. But alas what happened was that Sumo shut down production. I remember reading something in their history about this but don't remember exactly why it happened. I think the designer passed away at some point if I am correct. So though these were nice sounding amps they haven't been available for a long time. I have some Parasound amplifiers that just never had the full bodied sound of the Sumo. Actually I have an older Harmon Kardon amplifier that sounds better than the Parasounds. Now to find out if anyone has ever cloned one of these amplifiers? I'll have to put up a post and ask that question.

Steven
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