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 Post subject: Speaker level converter
PostPosted: Sep Mon 20, 2021 1:32 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 21406
Location: Warner Robins, GA
I have a Lafayette LA-375 amp that I modified to where the remote speaker jacks are an output for a powered sub with speaker level inputs. I used a 100uF cap per channel to couple the output to the output stage so that I would have zero phase shift. The amp drives speakers through the stock speaker coupling caps with a 15000uF cap in parallel so that I have zero phase shift to 70Hz.

What I am wanting to do is add a second amplifier to drive another pair of speakers with the amp input being fed by the subwoofer output I added to the Lafayette.

I have a Curtis Mathes HA235 amp that I want to use.

What I need is a resistive divider network to drop the signal level to what the amp input can handle.

The only problem is I don't have any specs on the amp so I do not know its input impedance and thusfore I do not exactly know what resistors I'd need for the divider.

I suppose that I could use a single series resistor in series with each input and possibly could use a resistance substitution box to figure the proper resistor value.

That however will require modifying the Curtis Mathes amp though. I'd have to cut two PC board traces on the line input jack so I can insert the resistors and I'd have to either use RCA Y cables or cut two more PC board traces to the phono input and wire the line input jacks straight to the phono input, although in that case I'd just unsolder the wires from the phono preamp and input jack board.

The Curtis Mathes specifies 8 ohms or more speaker impedance, although at home at one point I had it powering two sets of 8 ohm speakers in parallel without a single issue.

I'd only need a small pair of 8 ohm speakers though.

Also that would require me to modify the Lafayette so that the unswitched outlet is switched so the Lafayette can turn the other amp on and off given I modified the Lafayette with a relay across the power switch so that an external audio selector/vu meter with remote can power the Lafayette on and off and the switched outlet has a DAC plugged into it.

Would it be best to use a voltage divider with two resistors (One series and one in parallel with the input) or a single resistor?

Also is there a better way to accomplish what I am wanting to do?

Should I try to find another Lafayette LA-375 amp in order to keep the sound the same and eliminate the need to modify the existing Lafayette's unswitched outlet? Also the Lafayette would be easier as it already has series resistors on the two inputs so I could add a parallel resistor to ground on each channel to set the level properly. I could them simply remove the wires to the record out jacks and use the jacks as a pass through.

I'm kind of hesitant to modify the Curtis Mathes as I have the matching tuner, but will do it if it will work.

I could get six RCA jacks and a project box and use that for the resistors.

EDIT:

Gave it some thought and getting another Lafayette amplifier is the best way to go about it in my opinion.

One reason is that technically the only necessary mod I need to do is increase the value of series resistors on the input. I'd then use RCA Y cables to send the signal to the amp and plate amp.

EDIT:

A Lafayette LA-424 amplifier may be the easier way to go as it has a four channel preamp section with a two channel power amp. So it could drive the LA-375 directly from the rear channel variable line level outputs.

The only issue is it's short two speaker jacks. I could easily make do with the main speaker outputs and the remote speaker outputs could feed the powered sub which would require disconnecting the remote speaker jacks from the main/remote switch and doing the capacitor mod like I did to the LA-375.

The only issue is this.

Given the LA-424 will have either a 4 or 8 ohm speaker load it would have a slightly different amount of phase shift at 70Hz for the same value of speaker coupling capacitor as the LA-375 is driving a load of 2.6667 ohms as the speakers on it are 8 ohms and 4 ohms. How it will affect the integration with the powered sub I don't yet know.

I cannot remember how the amp is set up though. I had one as a teen and seem to remember maybe the rear channels are what's connected to the built in amp.

Thinking about it I may not have to mod the LA-424 at all as I can simply use the LA-424 as the amp for the third pair of speakers and the 375 can still be for the main two pairs of speakers and powered sub.

What prompted this is where I work it's hard for the audio to be heard at a test set near me that I and a co-worker operate without turning the main two speakers on my desk up loud enough to be a bother to others.

I currently have the other two speakers turned towards the test set, but I miss how much better the audio sounded with them turned towards me.

I have two small speakers on my desk and the other two smallish speakers were sitting on a 5 drawer filing cabinet on either side of my desk. So it seemed to give a wider sound stage.


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