Italian oddbal LEM G201 (trying to repair/mod)

[Superfuzz]

Hi guys! I got this early 80's amp made by LEM
It sounded nice back then but it was too underpowered and running it at max busted the output section so many time years ago, so my mates stopped using it and it got shelved...

I decided to give it another chance but I want to replace the output board with something more efficient..maybe D-class??
I measured the transformer voltage and I got +/-32-35V wich is not that much..

The original has two BDW52C/BDW51C couples and BD317S/BD318S as drivers..

ideas??

it looks like this!

[phatt]

http://sound.westhost.com/project27.htm
You just need the power stage down the page a bit.
Rod sells kits for this if that is the way you wish to go.
ESP has a good reputation and rightly so it's good gear.
The voltage you have is close enough but you will need to check if your transformer can deliver the current needed.
You may have to upgrade that to get the job done. 8|
Phil.

[Roly]

Rod's Project 27 isn't Class-D but it's hard to go past.   :dbtu:

The voltage you have is close enough but you will need to check if your transformer can deliver the current needed.
You may have to upgrade that to get the job done.

Surely not if he's using the same cab/load?

[teemuk]

I measured the transformer voltage and I got +/-32-35V wich is not that much..

Ignoring effects of loading and voltage sag that voltage range is good for an amp of about 60W - 100W to four ohms.

But that assuming the power supply can sustain that voltage when a lot of current is drawn.

The voltage you have is close enough but you will need to check if your transformer can deliver the current needed.

This. If the current rating isn't up for the job and the voltage just drops like no tomorrow as soon as the amp is loaded then the power amp obviously has much less to work with.

Voltage rating is only one part of the equation. Current rating is the second.

[Superfuzz]

Dunno about the current rating (you're talking about VA right?) but the transformer is quite big if this means something

No  indications are written on the chassis or over the trafo..

[phatt]

Rod's Project 27 isn't Class-D but it's hard to go past.   :dbtu:

The voltage you have is close enough but you will need to check if your transformer can deliver the current needed.
You may have to upgrade that to get the job done.

Surely not if he's using the same cab/load?

Urr? Unless I'm mistaken the fuses on Superfuzz's picture just posted Clearly state 1A primary and 2.5A on the secondary windings.
Rods circuit shows 2A primary and 5A secondary which makes sense as P27 has 4 output devices whereas the LEM has only 2.
Super fuzz noted it did not have enough power so I figured a New power transformer might be needed.
Phil.

[Superfuzz]

well the Lem has, 4 output devices as well (2X couples of BW51C/52C) but as you say, it seems that the whole trasformer is scaled down to one half of what I need...

[Roly]

GUYZ!  What determines the maximum current drawn from the supply, the amplifier, or the LOAD?

If the load/cab is the same/unchanged the amp in between makes no difference.

Right?

If you want more power into a given load then you first need more voltage, then maybe more devices to handle the higher resulting current.

Right?

{or am I having a seniors moment here?}

[teemuk]

If you want more power into a given load then you first need more voltage, then maybe more devices to handle the higher resulting current.

Right?

Power is a function of both voltage and current. A successfull design needs a proper reserve of both.

Yes, voltage is needed. There is no question of that.

BUT... current is also needed. If the power supply can't provide enough current the power supply voltage sags.

So that brings us again to having less voltage for the amplifier to do its thing. That's the reason why supply voltages are sometimes indicated in both unloaded and loaded conditions.

See where this is heading...? Theoreticallt you could build an amplifier that could supply all the current in the world but that amplifier isn't magically creating that current. It's just a "valve" between the power supply and the load. Ultimately it all falls down to power supply and its capabilities.

So your amp has too low supply voltage to produce 100 watts across 4 ohms? Well, use 2 ohms. There's your supply voltage restriction.

So your power supply can only provide 100 milliamps of current? Yeah, let's see what you can do to that.

[Roly]

Yeah, well that looks like about a 200VA core to me.

What am I missing here?

Let's go back to the OP.  If the amp is "underpowered" then that is a function of the power supply capacity, and changing the power amp to Class-D, Project 27, or anything else, isn't going to make any difference at all; the power supply is delivering what it can into the load (apparently without catching fire).

Changing the power board might make it more reliable (if the basic cause of blowing transistors is original mis-design; possible but unlikely), but things like faulty speaker leads or connectors have to be considered and eliminated first.

Currently it has "two BDW52C/BDW51C couples".  These are each 100V/15A/125W transistors and it has 2 pair - so why has it "busted the output section so many time(s)"?  At face value these should be more than capable of delivering 100W into 4 ohms (given sufficient heatsink).

Upping the power output would require some major changes, such as a heavier power transformer (but to me that core looks quite sufficient for 100W output); but again I would want to eliminate the possibility it has inefficient speakers and could possibly be improved by changing the speakers for more efficient ones.

Now if it has the original power supply and the original speaker load it can only perform as original, and it seems the problem here is blowing transistors, not blowing the power supply, so the supply must therefore have sufficient capacity to drive the original load.

Until we change the basis of this discussion to "more volts" or "fewer ohms" the power supply won't need changing.

220v * 1A = 220 watts in, about what you would expect for a nominal 100W output amp.

35VDC implies ~25VAC * 2.5A = 62.5 watts.  But it has two such secondary fuses and there would seem to be no reason for that with a single winding into a bridge, so (without a circuit) we can guess that it has two fuses because it has two secondary windings to protect, and that the available DC power will be around 125 watts.  Fuse blowing isn't the complaint.

So;
- what is the original nominal output power?

- what is the speaker cab impedance? (or simply measure the cab ohmmeter resistance and post that)

[phatt]

Urrh huh,, ta Roly a little light bulb just lit up somewhere in my head.  :dbtu:

Ok 4 output devices and 2.5 amp fuse,, so I'm now wondering why P27 has 5 Amp fuses?
Head scratch??
I'm thinking it may have to do with the design,, tolerance of a particular type of circuit?
Phil.

[Superfuzz]

Yeah, well that looks like about a 200VA core to me.

What am I missing here?

Let's go back to the OP.  If the amp is "underpowered" then that is a function of the power supply capacity, and changing the power amp to Class-D, Project 27, or anything else, isn't going to make any difference at all; the power supply is delivering what it can into the load (apparently without catching fire).

Changing the power board might make it more reliable (if the basic cause of blowing transistors is original mis-design; possible but unlikely), but things like faulty speaker leads or connectors have to be considered and eliminated first.

Currently it has "two BDW52C/BDW51C couples".  These are each 100V/15A/125W transistors and it has 2 pair - so why has it "busted the output section so many time(s)"?  At face value these should be more than capable of delivering 100W into 4 ohms (given sufficient heatsink).

Upping the power output would require some major changes, such as a heavier power transformer (but to me that core looks quite sufficient for 100W output); but again I would want to eliminate the possibility it has inefficient speakers and could possibly be improved by changing the speakers for more efficient ones.

Now if it has the original power supply and the original speaker load it can only perform as original, and it seems the problem here is blowing transistors, not blowing the power supply, so the supply must therefore have sufficient capacity to drive the original load.

Until we change the basis of this discussion to "more volts" or "fewer ohms" the power supply won't need changing.

220v * 1A = 220 watts in, about what you would expect for a nominal 100W output amp.

35VDC implies ~25VAC * 2.5A = 62.5 watts.  But it has two such secondary fuses and there would seem to be no reason for that with a single winding into a bridge, so (without a circuit) we can guess that it has two fuses because it has two secondary windings to protect, and that the available DC power will be around 125 watts.  Fuse blowing isn't the complaint.

So;
- what is the original nominal output power?

- what is the speaker cab impedance? (or simply measure the cab ohmmeter resistance and post that)

- well, its original nominal output power is un-mentioned on the chassis, back in the days , when I was not involved with electronics, the tech said it was like 70W..
- the cab instead was 8 ohm, wich makes sense of why it blew so many times..most of our cabs are 4 ohm, and this head has litteraly no heatsink apart from the chassis itself..

The output section looks very strange anyway, I got to post a pic.. it has a quad op-amp, at its center and it includes the power supply capacitors wich are 4 and each one is located next to one output transistor..

I'm a newbie here, I'm a DIY fuzzbox maker, but it looks to me like faulty design/unrealiable..probably not so efficient too..

[g1]

  Would a class D power amp not be more efficient?  Wouldn't that translate into more power to the load from a given supply?

[Roly]

the tech said it was like 70W.

Which is exactly in line with my guess of somewhere between 60 and 80 watts.

- the cab instead was 8 ohm, which makes sense of why it blew so many times..most of our cabs are 4 ohm, and this head has literally no heatsink apart from the chassis itself..

Most certainly if you try to drive a 4 ohm load with an amp designed for a minimum 8 ohms you are going to have problems.

Two pairs of these transistors should, at face value, be able to cope with a 4 ohm load, BUT if the heatsinking is inadequate (and I don't normally consider a chassis to be an adequate heatsink) then I would guess that this is the basic cause of the output transistors giving up - overheating.

Would a class D power amp not be more efficient?  Wouldn't that translate into more power to the load from a given supply?

Interesting point.  It's an inviting idea, but the way I see it we basically have a battery (power supply) connected to a resistor load (loudspeaker).  The power in the load is determined by the  voltage available (the rail voltages) and the load impedance.  A Class-D stage may not waste as much power as heat, but the only gain would be in less power supply sag which would only be a marginal increase in available output power.  Even if the amplifier in between is 100% efficient it still comes down to available power supply voltage across the load impedance.


In this case I'm firming up on the idea of poor speaker efficiency.

[J M Fahey]

1) That amp was made by LEM which is an experienced Amp company with decades of amp production.
Mainly PA but that experience applies to guitar amps too.

2) that amp has worked before, as all its brothers, it blew once for unknown reasons, like *any*  amp can (that's why such things as Service, Warranty, etc. exist), and after that, if it blew again *many*  times it was either improperly repaired, misused/abused or both , .

3) a new faucet won't pull more water from the wall (the PSU) into a bucket (the speaker load).
The amp is that faucet.

4) also agree that the amp is roughly 70W into 8 ohms, and for the power the transformer is overengineered (i.e. more than enough).

5) just one pair of those output devices is more than enough for 70W/8r , 2 pairs are overkill, so lack of devices is not the point.

6) using the chassis is fine, as far as said chassis is aluminum and not iron , such as exemplified by thousands of Fender combo amps, Acoustic, the unsinkable Randalls, etc.
Of course, if that amp was designed and tested with 8 ohms loads and now is used with 4 ohms ones, that might "void the warranty  :lmao:

7) For practical reasons (mounting, space, availability, etc.) I'd rather repair that same power amp pcb *and* add a flatback finned heatsink block to the other side of chassis, screwed tightly down for good thermal contact and with thermal grease inbetween.


Busy now finishing a 300W Bass amp, by the way with output MosFets bolted to the aluminum back panel, go figure ..... although I "cheat" by using a fan

*If* the (usually very impatient) customer allows it, I'll take and post a couple pictures.