Homebrew guitar amp, speaker mismatch

[sirdavy]

I'm making a battery powered guitar amp out of some spare parts. I have the output/amp stage of a battery powered radio (Hacker Rp34 Democrat) which I am going to connect to a single speaker cabinet (Wharfdale Linton II). In the portable radio the amp normally powers a 15 ohm speaker, the Wharfdale is only 6 ohm, so there's a bit of a mismatch.

I have two 4 ohm/25w resistors that I can put in series between the amp and the speaker - 4+4+6 = 14ohms - and that should even things up.

However, I want to have some sort of master volume in the form of an L-Pad. Would an 8ohm L-pad in conjunction with the two 4 ohm resistors work for the mismatch?

Would the best configuration be from amp's output positive wire:

4ohm resistor - 4ohm resistor - L Pad input - L pad output - speaker?

or

L Pad input - L-pad output - 4ohm resistor - 4ohm resistor - speaker?

or something else?

Or should I use a (more expensive) 16ohm L-Pad - would that fix the mismatch?

[peter_sol]

You will lose around half of your output with the resistors. And/ or pad. I think you will be fighting for gain as it is. So may need a preamp in front of the output amp.
It needs 2, 9v batteries and produces 1.25 watts for an input of 70 mv.

[Ed_Dinning]

A small matching transformer, turns ratio 1.41 to 1 would be a better bet

Ed

[Oldcodger]

And ( if you have not looked into the subject) look at speaker balance/phasing. it's where the cone of one speaker is goes forward, but the other one ( of a stereo pair) goes backward. Simple method is to connect a low voltage source to each and see which direction th cone moves.

[sirdavy]

Originally Posted by peter_sol

You will lose around half of your output with the resistors. And/ or pad. I think you will be fighting for gain as it is. So may need a preamp in front of the output amp.
It needs 2, 9v batteries and produces 1.25 watts for an input of 70 mv.

That’s OK, I’m not looking for it to be efficient. I’m powering it from a usb powerbank with a buck converter module to get the 18v. The project is supposed to result in a kind of Brian May Deacy amp (which he plays with a treble boost pedal in front of, so I’m expecting to put something inline beforehand). My Linton cabinet is like the one used for the Deacy amp in that it has a woofer and tweeter but the Linton has a crossover circuit whereas the (in the attached pic) the Deacy appears to have a single capacitor joining the woofer and tweeter.

Yesterday I lashed it all together with just the resistors in series and the guitar came through loud and with an interesting distortion but the volume wavered oddly, with a sort of slow tremolo effect. It didn’t do this with the radio’s original 15ohm speaker, and the Linton cabinet didn’t do it when taking a feed from another battery guitar amp I have. Not sure why…

[sirdavy]

Originally Posted by Ed_Dinning

A small matching transformer, turns ratio 1.41 to 1 would be a better bet

Ed

Would this work? Do I put it between the amp and the speaker?

I really need something to attenuate the volume too, could I put an L-pad between this new transformer and the speaker?

[sirdavy]

Originally Posted by Oldcodger

And ( if you have not looked into the subject) look at speaker balance/phasing. it's where the cone of one speaker is goes forward, but the other one ( of a stereo pair) goes backward. Simple method is to connect a low voltage source to each and see which direction th cone moves.

I haven’t checked this, didn’t know hifi speakers could be set up like this. Will check with a 9v battery.

[Simon Gittins]

1.5V is all you need, 9V might be too much for some speakers.

[Ed_Dinning]

Hi Davy, transformer is far too puny I'm afraid

Ed

[sirdavy]

I'm having difficulty finding a suitable audio impedance matching transformer that will allow an amp that normally has a load of 15ohms powering speakers of 6ohms. None of RS, Farnell or Mouser seem to sell anything with that step-down but I might just be misunderstanding how they are rated.

I might just go with an L-Pad. What would be the correct setup for 15 ohms amp connected to a 6ohm speaker - an 8ohm l-pad or a 16ohm l-pad?

There is no real matching twixt amplifier and 'speaker here just the maximum current capability of the amplifier. This case 18V divided by two (peak to peak AC), 9V into 15 ohms, 600mA, older germanium transistors did voltage but not much current hence 18V and a high resistance 'speaker A 'speaker with a lower resistance will demand more current, 6 ohms would like 1.5A. Yor best bet would be two 8 ohm speakers in series, or a 9 ohm resistor (8 will do) in series

[sirdavy]

Originally Posted by merlinmaxwell

There is no real matching twixt amplifier and 'speaker here just the maximum current capability of the amplifier. This case 18V divided by two (peak to peak AC), 9V into 15 ohms, 600mA, older germanium transistors did voltage but not much current hence 18V and a high resistance 'speaker A 'speaker with a lower resistance will demand more current, 6 ohms would like 1.5A. Yor best bet would be two 8 ohm speakers in series, or a 9 ohm resistor (8 will do) in series

Thanks. I have two 4ohm resistors i can put in series, as you suggest, but I'm still looking for an Lpad to use as a master volume. Should it be a 16ohm or 8ohm l-pad? Should I use one in conjunction with the resistors or will the l-pad be enough on its own?

Could I solve the mismatch by reducing the voltage into the amp down to 9v? (My buck booster can adjust down to this).

Lpad to use as a master volume

That should be in the input of the amplifier, a simple 10k pot will do, this stops the amplifier going all out all of the time, saves power too.

[sirdavy]

Originally Posted by merlinmaxwell

That should be in the input of the amplifier, a simple 10k pot will do, this stops the amplifier going all out all of the time, saves power too.

I'm making a guitar amplifier and it going all out some of the time - or all of the time - is exactly the sort of attribute I'd like to have available. I'm going to have a pot attenuating the input from the guitar, I also want a master volume on the speaker, so I need to figure out which L-pad (and possibly resistors) i need to have. Any ideas?

[sirdavy]

The woofer and tweeter inside the hifi speaker separately measure 7.6ohms and 13.4ohms (using a DMM). I'm guessing they are wired in parallel and along with the crossover unit inside result in the 6ohm load.

Would it be possible to fix the mismatch by wiring the woofer and tweeter in series for a 20ohm load (assuming a higher load is a better mismatch for the amp)? Can you wire a woofer and tweeter in series?

[buggies]

My suggestion would be to use an 8 ohm L-pad for example:-
https://cpc.farnell.com/monacor/lp-1...543?st=l%20pad
Which costs £6 plus postage. A brief data sheet is avalable at that page.
This presents a (relatively) constant 8 ohms to the driving source - assuming an 8 ohm speaker is connected.
To keep your amplifier happy you will need to add the 8 ohm resistor between the amplifier and the L-pad so that it always sees about 16 ohms.
As mentioned earlier this will lose you half the power output but would work until you upgrade the amplifier (to lower impedance output) or locate a suitable transformer.
I would not attempt to series connect a tweeter and woofer.

[sirdavy]

Originally Posted by buggies

My suggestion would be to use an 8 ohm L-pad for example:-
https://cpc.farnell.com/monacor/lp-1...543?st=l%20pad
Which costs £6 plus postage. A brief data sheet is avalable at that page.
This presents a (relatively) constant 8 ohms to the driving source - assuming an 8 ohm speaker is connected.
To keep your amplifier happy you will need to add the 8 ohm resistor between the amplifier and the L-pad so that it always sees about 16 ohms.
As mentioned earlier this will lose you half the power output but would work until you upgrade the amplifier (to lower impedance output) or locate a suitable transformer.
I would not attempt to series connect a tweeter and woofer.

Great, I'll try that. (I'm not going to upgrade the amplifier btw, I specifically want to mess about with this old amp and see what odd guitar tones I can get from it).

Having lashed up the amp and hifi speaker I reckon something's up with the crossover. Both woofer and tweeter responded to a 1.5v battery check but playing the guitar through it the lower/bassier strings ring loudly but the higher/treble strings hardly make a noise. Have got some replacement capacitors on the way, along with the L-pad.

[Silicon]

The low frequencies can damage tweeters.

The tweeter won't last long if you wire it in series with the woofer.

[sirdavy]

Originally Posted by Silicon

The low frequencies can damage tweeters.

The tweeter won't last long if you wire it in series with the woofer.

I'm probably not going to go down this route but I've read on a few hifi forums that wiring them in series is possible with a capacitor (2.2uf?) in series with the tweeter.

[G.Castle]

Originally Posted by sirdavy

... I've read on a few hifi forums that wiring them in series is possible with a capacitor (2.2uf?) in series with the tweeter.

All that would do is stop much bass reaching the speaker at all.