Tweed Champ Line Out

[mcgruff]

I'm planning to buy one of these Tweed Champ kits in the next couple of weeks.

However, I'd like to add a line out so I can plug straight in to the DAW. Handy for late night practicing on headphones but maybe also for recording if I can get a decent sound.

I know the power amp stage is a big part of the character of an amp, and I'm assuming the output transformer is too, so presumably the best way to do this is to use the speaker out signal, take the line out from here (somehow...), and add a dummy load - preferably a reactive load. This RAT Dummy Load is very close to what I'm looking for but it's a resistive load.

Anyone done this before? I'm hoping someone has heard of a circuit diagram to do this. If not, and you help me build one, I promise to write up a blog with all the details which will serve as a reference for anyone else looking for a line out which preserves as much of the amp's original tone as possible

[olddawg]

I'm planning to buy one of these Tweed Champ kits in the next couple of weeks.

However, I'd like to add a line out so I can plug straight in to the DAW. Handy for late night practicing on headphones but maybe also for recording if I can get a decent sound.

I know the power amp stage is a big part of the character of an amp, and I'm assuming the output transformer is too, so presumably the best way to do this is to use the speaker out signal, take the line out from here (somehow...), and add a dummy load - preferably a reactive load. This RAT Dummy Load is very close to what I'm looking for but it's a resistive load.

Anyone done this before? I'm hoping someone has heard of a circuit diagram to do this. If not, and you help me build one, I promise to write up a blog with all the details which will serve as a reference for anyone else looking for a line out which preserves as much of the amp's original tone as possible

It's usually just an 820 ohm resister in series with a jack isolated from ground in parallel with the speaker. Basically a 100/1 voltage divider. You will find that you will have a lot of hiss though. When you attenuate a speaker level signal the noise floor really goes up. Sometimes it doesn't matter though when recording. A big noise floor can cover up a lot of evils. Look up Randy Bachman's Hertzog to see how it was done in the old days.

[mcgruff]

The Herzog looks interesting - crazy sound in this vid

Noise floor: is this because the signal level (voltage) has been drastically reduced but the noise level stays the same? There must be a way to do this which keeps the noise down and preserves the amp tone.

What about a reactive load - same problem?

[olddawg]

The Herzog looks interesting - crazy sound in this vid

Noise floor: is this because the signal level (voltage) has been drastically reduced but the noise level stays the same? There must be a way to do this which keeps the noise down and preserves the amp tone.

What about a reactive load - same problem?

When you re-amplify the attenuated signal you re-amplify the noise. If the attenuation is 100/1 it's 100 times as much noise back to unity plus whatever noise floor the second amp has. A reactive load just sounds a little different to some people. I use a Weber Mini Mass attenuator which has a speaker motor reactive load and a lot of other features like treble bypass. You can get them with a line out, foot switchable, etc. Best $125 I have spent in a long time.

[mcgruff]

OK it looks like it's just a bad idea to use the speaker out.

Would the solution be a transformer with a special line out section as well as the speaker out?

[Tom Phillips]

When you re-amplify the attenuated signal you re-amplify the noise. If the attenuation is 100/1 it's 100 times as much noise back to unity plus whatever noise floor the second amp has. ...

My take is that the attenuator would lower the noise and the signal by the same amount. If you re-amplify the attenuated signal I understand about the noise floor of the second amp but you totally lost me about "...it's 100 times as much noise back to unity..." It seems like the signal to noise ratio would not be a problem. Especially if you just attenuated down to line level. Am I missing something in your post?
Regards,
Tom

[olddawg]

My take is that the attenuator would lower the noise and the signal by the same amount. If you re-amplify the attenuated signal I understand about the noise floor of the second amp but you totally lost me about "...it's 100 times as much noise back to unity..." It seems like the signal to noise ratio would not be a problem. Especially if you just attenuated down to line level. Am I missing something in your post?
Regards,
Tom

Maybe I wasn't to clear with that. Yes you will amplify the noise and the signal equally. But there is always much more noise. Especially hiss when you attenuate a speaker level signal down to line level and then re-amplify it.

[mcgruff]

I found this "thrifty line out" on wattkins.com. Looks interesting - independent volume and tone controls for the line out. A guitar speaker acts like a low pass filter so the tone could be very useful to cut some highs.

I'd also need to add a dummy load. This DIY reactive load mentioned in the above thread looks very well designed.

[Tom Phillips]

Maybe I wasn't to clear with that. Yes you will amplify the noise and the signal equally. But there is always much more noise. Especially hiss when you attenuate a speaker level signal down to line level and then re-amplify it.

Agreed. The noise was always there. It is just not reproduced by the guitar amp speaker as mcgruff mentions in post #8.

I found this "thrifty line out" on wattkins.com. Looks interesting - independent volume and tone controls for the line out. A guitar speaker acts like a low pass filter so the tone could be very useful to cut some highs. I'd also need to add a dummy load. This DIY reactive load mentioned in the above thread looks very well designed.

It’s best to use a “compensated line out” circuit. That’s just the attenuation plus filtering to simulate that provided by the speaker. Then you can always add variable EQ to allow final adjustments to the sound. Otherwise, as olddawg points out you will hear much more of the high frequency noise.

I think you are on the right track with the reactive load and additional EQ approach. I have been thinking about doing the same thing to make a silent practice system. I’m interested to hear how this works out for you.

Cheers,
Tom

[mcgruff]

I'll keep you posted. Should be ordering up the kit & parts as soon as the April paycheque comes in.

A Tweed Champ head with a good line out could be quite a useful little box for recording and performing as well as practicing.

[Tom Phillips]

...A Tweed Champ head with a good line out could be quite a useful little box for recording and performing as well as practicing.

I was actually considering making a standalone reactive load/attenuator/eq unit then drive headphones directly or feed a mixing board so I could mix in music or tutorials to play along with. I think the eq would be an important part of the process. The bands would need to be adjusted to mimic speaker roll off. The standalone box could then be used with different amps.

[mcgruff]

Just put in an order to ERSE:

200µF 250Vdc PulseX Cap - Metallized Polypropylene Film Capacitor
ESQ55-16-10000 10.0mH 16ga ESQ - Super Q Coil
EAC32-18-500 0.5mH 18ga EQ -Perfect Lay High Bond Air Core Inductor

These are the same parts in the other thread, all rated for a 100W amp. You could probably get the cost down by sourcing lower-spec parts (see the other thread page 10) but these are proven to work well so I just went ahead and ordered them.

These cement resistors from ERSE are quite cheap and 10W should be enough for a Champ:

EWR10-05-8.0/PB 8.0Ω 10W wire wound resistor
EWR10-05-50.0 50.0Ω 10W wire wound resistor
EWR10-05-82.0/PB 82.0Ω 10W wire wound resistor
EWR10-05-25.0/PB 25.0Ω 10W wire wound resistor

Some of these parts have to be back-ordered, plus I'm in the UK, so now for a long wait...

[olddawg]

The weber Mini Mass has a reactive load and a compensated line out: Attenuators...... And is around $100 depending on your options and wattage. I got the 50 watt version with the foot switch option for $125. I couldn't build it for that.

[mcgruff]

The Weber mass series are pretty good but only for small amounts of attentuation. With a full-blown dummy load there's no limit to how low you can set the final volume via a slave or PA, or you can even have no volume at all if you simply want to practice silently on headphones at home. Anything where attenuation alone can't reduce the volume as much as you need.

[Tom Phillips]

The Weber mass series are pretty good but only for small amounts of attentuation...

They must have changed the design because the Weber site now describes silent playing and headphones only features. There is a statement that you can use the attenuator set to zero output and unplug the speaker entirely.

It appears that they no longer sell the standalone speaker motor for those who want to roll their own.