Adding a Line Out

[mcgruff]

I'm building a Matchless Spitfire/Lightning and this is the line out:



If I wanted to change to 16ohm and 8ohm would I need to change the resistor values? I don't know how to work this out but if I had to guess I'd double the 22k to 44k (or close - doesn't have to be perfect) but keep the 3k3.

Also would it be a good idea to fit a ground lift switch (cutting the connection from the 3k3 to the amp's ground)?

Thanks.

[Chuck H]

Add a pot to make it infinitely adjustable for actual gain. Change the 22k to 2.2k and change the 3.3k for the outside lugs of a 500ohm pot with the wiper to the output jack. This will allow you to dial in a stronger signal at lower gain settings and down to no signal for higher gain settings. No need to ever lift the circuit because the impedance is so high compared to the speaker load that it has virtually no affect on the output impedance.

EDIT: Re reading your post there's a very easy answer... If you're happy with the performance now, just leave it the same and keep the line out circuit connected to the 8 ohm tap. The voltage will remain the same as what you have now. The line out circuit operates at the tap voltage it's wired to regardless of which tap the speakers are using.

[mcgruff]

That's great Chuck - thanks

[Gingertube]

And I generally add one of these to isolate it and give you the option of balanced line out - not required but it is nice. I put a 25K pot across its secondary, wire one secondary side to 0v via a 51 Ohm 1W resistor and a 10nF 1kV ceramic in series. That is I maintain DC galvanic isolation.
The tranny will cope with up to 10V RMS. If wiring to high power amp you might want a voltage divider on the input side.
M0710 10k Ohm / 10k Ohm Bridging Input Transformer - Altronics
Cheers,
Ian

[Chuck H]

Times are changing. Who knows what someone may need to plug into in this age of home recording. Isolation is never a bad thing when the only downside is a few bucks.

I've never included an isolation transformer, but I've always designed for stage use and my old school mentality keeps me thinking that the studio is going to be a miniature version of that with tapes running Not at all so any more. A balanced output and especially the isolation would be worth the price of admission with modern considerations.

[mcgruff]

And I generally add one of these to isolate it and give you the option of balanced line out - not required but it is nice.

My recording gear is all balanced so this would be a useful upgrade. Do you think there's any point using a high-quality transformer like the Jensen JT11P1 in a guitar amp?

[patlaw]

My recording gear is all balanced so this would be a useful upgrade. Do you think there's any point using a high-quality transformer like the Jensen JT11P1 in a guitar amp?

It depends on who you ask. A cheaper transformer will alter the sound more than a better one will, but the sound of the Line Out is so different from the sound out of the speaker that the choice of the transformer is probably not a big deal. It's amazing how good some of the cheap transformers are these days.

I do have one thought, though. I would tend to use a 600:600 transformer with a 600 ohm resistor in series with the primary. If there is a long cable run on the line out, a 600 ohm resistor at the input to the mixer could be useful, assuming the line input on the mixer is high impedance. Having said that, one major transformer manufacturer told me that their 600:600 and 10K:10K transformers are the same. They're just labeled differently for people like me. I don't know if that practice is the same for all transformer manufacturers.

[mcgruff]

Yeah the raw amp output is unusable: it's like dazzlingly bright, harsh sunlight. A speaker adds a stained glass window in front of that to filter the glare down into something bearable.

The line out will let me record the raw amp tone at the same time as a mic'd cab. The mic'd cab would always be the main sound but I'd also have some scope to tweak the tone at the mixing stage by mixing in the line out signal after processing in a convolution cab sim. You could maybe add a little extra brightness or bite, create a stereo effect, or whatever.

It would also be interesting to compare the raw signal from my DIY reactive load with the raw signal from the line out where a real speaker is providing the load. That would let me evaluate how well the load is working - maybe I need to tweak some component values.

The reactive load is based on the Aiken paper - let me know if anyone wants more info on the circuit.

[Chuck H]

I've built a tuned filter circuit for headphone practice. I either never could get it to sound right or I just think it sounds like crap anyway I know that on some models Matchless used a .047 cap across the load resistor on the line out. Not sure how much it would help but they must have recognized the same harsh, bright tone you describe.

[mcgruff]

My "quiet" setup for practice/recording when I can't crank the amp is speaker out -> reactive load -> computer -> convolution cab sim. With some good impulse response files (Ownhammer are the best I've found) you can get a pretty realistic sound.

The reactive load was pretty simple to build (could probably do with a balanced out there too):

[eschertron]

A cheaper transformer will alter the sound more than a better one will, but the sound of the Line Out is so different from the sound out of the speaker that the choice of the transformer is probably not a big deal. It's amazing how good some of the cheap transformers are these days.

And I generally add one of these to isolate it M0710 10k Ohm / 10k Ohm Bridging Input Transformer - Altronics

Good link! I'll have to try one, now that it's been suggested.

I had done some research while putting together a 12AT7-powered 'living room' amp. I couldn't bring myself to spend U$70 or so for the iso transformer (the pro-audio units I looked at) when I hoped to finish the whole chassis for around U$100 or so. For ten bucks I'll give it a go.

To set the transformer up as balanced line out, do you simply float the secondary's ground? I don't see a CT in the picture.

[patlaw]

I had done some research while putting together a 12AT7-powered 'living room' amp. I couldn't bring myself to spend U$70 or so for the iso transformer (the pro-audio units I looked at) when I hoped to finish the whole chassis for around U$100 or so. For ten bucks I'll give it a go.

To set the transformer up as balanced line out, do you simply float the secondary's ground? I don't see a CT in the picture.

The transformer is not shielded. Connect the frame to audio ground. The ground on the Line Out cable can connect to the audio ground of the circuit.

If that transformer picks up noise, you may have to go with a shielded one.

[Gingertube]

Secondary ground is wired to 0V via a 51 Ohm 1W resistor plus 10nF 1kV ceramic cap in series. This method copied from KOC (TUT5 "Standard" Amp).
Cheers,
Ian