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 Post subject: Lafayette LA-375 amp
PostPosted: Sep Fri 20, 2019 3:47 pm 
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Location: Warner Robins, GA
I am purchasing another Lafayette LA-375 amp which I will restore and use at work.

I plan on giving it a full recap and replacing the trimmer resistors.

C123 is a 25 volt rated part according to the schematic and the listed voltage at that point is 30 volts so I will replace it with a 35 volt cap.

The initial plan was to take it, mod it like my other one and use an Edcor XSM 15K/15K balanced/unbalanced to balancec/unbalanced transformer so I could have two monoblocks, but with what I work on now at work I needed a single speaker I could usae away from my desk.

Now should I just use one channel of the amp and set the stereo-mono switch to mono or should I get an Edcor WSM6400 and use the amp as a monoblock?

https://www.edcorusa.com/wsm6400

There any issues running the amp as a monoblock?


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 Post subject: Re: Lafayette LA-375 amp
PostPosted: Sep Fri 20, 2019 9:56 pm 
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Location: Sayreville, NJ 08872
Put it in Mono; add a 4 to 8 ohm resistor (several watts) to one channel; connect speaker to other channel; Done

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 Post subject: Re: Lafayette LA-375 amp
PostPosted: Sep Sat 21, 2019 12:15 am 
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I could but that would seem like a waste.

I could just use one channel perhaps with it in mono.

Not sure if the amp needs anything but caps and the trimpots though.

Setting the trimpots is to where I have 1/2 B+ at the speaker coupling cap + terminal and adjusting for 30mA idle current, right?


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 Post subject: Re: Lafayette LA-375 amp
PostPosted: Sep Sun 22, 2019 2:20 am 
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Location: Sayreville, NJ 08872
Tube Radio wrote:
I could but that would seem like a waste.

I could just use one channel perhaps with it in mono.

Not sure if the amp needs anything but caps and the trimpots though.

Setting the trimpots is to where I have 1/2 B+ at the speaker coupling cap + terminal and adjusting for 30mA idle current, right?


Remove either fuse in the back and you'll have a mono amplifier. Very simple.

For proper idling current of each channel, measure voltage at the cathode of the power supply diodes or at either fuse. Connect voltmeter to + terminal of speaker coupling cap and adjust RV104 (L,R) for half the voltage. Remove each fuse and connect ammeter across the fuse terminals. Adjust RV105 (L,R,) for 30 ma. Go back and check RV104 (L,R) adjustment.

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http://www.classicradiomanuals.com


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 Post subject: Re: Lafayette LA-375 amp
PostPosted: Sep Sun 22, 2019 3:07 pm 
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I may do just that with one channel.


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 Post subject: Re: Lafayette LA-375 amp
PostPosted: Oct Mon 07, 2019 10:01 pm 
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My plan is to first try this amp at work and see how I like it on the speaker I have there.

I currently drive that speaker with a Future Kit mono 20 watt amp and that sounds good and is very compact.

If the Lafayette doesn't sound good on that speaker I may mod it to be just like the other one I have. I will then upgrade to a dual winding torroidal transformer to power both amps and I'll take the two way 70Hz 24 db/octave active crossover and mod it for a balanced output which can be achieved by using a quad op-amp of the same type I already use for the crossover.

That would be done by using two op-amps as a buffer and non-inverted output and the other two op-amps as inverting op-amps driven by the two non-inverting op-amps. That way I have a balanced signal for the Lafayette amps which will then turn each amp into a 50 watt monoblock.

I'll either then stack the amp on top of the other or I'll put one amp on top of each of the two Electrovoice FR-200 speakers I use for my mains.


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 Post subject: Re: Lafayette LA-375 amp
PostPosted: Oct Mon 14, 2019 8:13 pm 
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The amp came in today and it is in good cosmetic condition only needing some light cleaning.

Not sure how it operates yet, but given the issue with the trimpots known to go bad I will not power it up until I order the parts to do a full recap along with new trimpots.

Still not sure if it will remain stereo or become a monoblock though.

EDIT:

Decided to power the amp after looking it over and not seeing anything visibly wrong.

I tried to adjust the trimpots and I think that went well, but I won't know until I use an actual ammeter which unfortunately I do not have at home at the moment.

Concerning the idle current adjustments is it ok to insert a .1 ohm resistor in series with each fuse permanently so that I can simply measure the voltage across the resistor to set the idle current or would that be too low of a voltage to get an accurate reading on my DMM that is only good to the nearest 100uV?

I do have a VTVM with a .5Vdc range so perhaps that could be used with the .1 ohm resistors.

Speaking of idle current I need to check the idle current of my other LA-375.

I do know that the idle current adjustment does affect the sound a little as when the idle current isn't right, the amp doesn't sound quite as good, but when the idle current is right, the amp sounds amazing. Hard to describe exactly what I mean and it may only be noticeable by someone with real good ears.

For driving the mono speaker given it has a dual concentric volume control where both sections adjust independently I can just leave one section turned down and just set the STEREO-MONO switch to mono.

Perhaps I could find an 8" or 10" speaker that I can put in a box with a choke in series and I could use the other channel to drive that speaker for better bass. Not looking to go to 20Hz, but I want better low frequency response than the 5" speakers give me.

EDIT:

I replaced the trimpots with multi turn trimpots like I did in my other LA-375.

I got the voltage and bias adjusted properly.

The way I did it was to use two ammeters and three voltmeters (did it at work) then adjusted the pots to the correct values and let the amp sit awhile then proceeded to readjust to optimum values.

I did notice some carbon comp resistors in the amp.

Are those known to go out of tolerance?


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 Post subject: Re: Lafayette LA-375 amp
PostPosted: Oct Fri 18, 2019 3:44 pm 
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I replaced all the carbon comp resistors with metal film resistors except the 270 and 680K as I had none of those.

Doing an initial re-check of the bias shows that the idle current is now approximately 10mA both channels so I had to re-adjust the bias.

Did an initial adjustment and will let the amp sit awhile powered up after lunch so it is up to operating temperature then I'll re-adjust the bias and voltage.

I'll also order the necessary resistors for my other LA-375 and replace them as well.

Sometimes it pays to have access to multiple DMMs as it sure has made adjusting this amp much easier.

I have two for current both channels, Two for voltage both channels and one for the B+ voltage.

EDIT:

Oddly enough most of the carbon comp resistors read lower in value.


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 Post subject: Re: Lafayette LA-375 amp
PostPosted: Oct Wed 23, 2019 2:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20416
Location: Warner Robins, GA
Recap is done and the carbon comp resistors have been replaced.

The 1uF caps I somehow got non polar caps and I somehow missed two 220uF caps.

It's good that I listened to the amp for awhile before the recap as I have a baseline to judge if the new caps improved the sound.

EDIT:

The amp seems to sound a little more clear than it was.


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