Running line outputs in parallel?

[riz]

Hi everybody, first post here and I have to say I've been thoroughly enjoying digging through the archives here! I have recently become obsessed with the idea of adding line out capability to my Fender Champ, a la Herzog. My prototype mod works and sounds good. I am about to perform a similar mod to an old Silvertone 1331, which was heavily modded before I recieved it (that's a whole 'nother thread in the making!) and has crushing amounts of gain. A truly spectacular amplifier. I would like to use these amplifiers together, and was originally planning to run them in series, into a larger amplifier. Now, my question is this: can I SAFELY run the line outputs of these amps in PARALLEL directly into the input of the third amplifier? Also, while I have your attention, can someone tell me why I see so many different pot and dropping resistor values for line out mods on the net? I've tried both a 5k (from a Soldano schem) and 1meg (Herzog) pot and can't really hear the difference. I'm good at parts-for-parts repairs, cap jobs etc., but am becoming increasingly interested in the theory behind it all in hopes of eventually being able to design my own mods. So close, yet so far...! Thanks in advance!

[Enzo]

TO go from one larger voltage of signal to a smaller one, we often use a voltage divider circuit. The operative word there is circuit. A voltage divider works on the ratios of the upper and lower resistances. What those resistances are is less important than their ratio.

Look at a common volume control. Turn it half way up. Now it doesn't really matter so much whether it is a 1meg pot or a 250k pot, half way up will be half way up. That is because the ratio of the upper half of the pot resistance to the lower is the same in each case. Oh there may be some subtle loading effects or something in some cases, but I doubt our line out circuit slapped across the output transformer of a tube amp will load it at all.


Mixing signal sources by connecting them together is not really a recommended practice.

[riz]

Ahh, the voltage divider... My brain only recently began to grasp this concept, and soon I hope to have the time to sit down and do some practice math using some familiar schematics. I'm glad you mentioned loading as well, that's another concept I'm slowly trying to understand in more depth. Like I said, I feel it will become clearer when I have done more homework. I was hoping that running these in parallel would be somewhat akin to channel jumper-ing. I'll leave that alone for now.

[J M Fahey]

You should get the simplest and cheapest of line mixers, those with, say, 4 input jacks and 4 volume pots, even a passive one, so you can mix properly and set the sound you like.
EDIT: channel jumpering is mixing inputs, and to boot, uses mixing resistors.

[Enzo]

Ohm's Law, voltage dividers - about the two most fundamental concepts in electronics and very widely applied. I have been soldering for over 50 years, and I use Ohm's Law every day. Seriously. I have a little calculator next to me, so I can figure the current through something or what resistor I need for some LED current limiting, or who knows. Ohm's Law is everywhere.

And voltage dividers are everywhere as well. You might find a volume control an obvious voltage divider, but when there is a 470k resistor to ground from some tube grid, and the signal comes in from the preceding stage through another 470k resistor, BAM! they just formed a voltage divider - they take the incoming voltage and cut it in half.

Learn your color code, and how to calculate resistors or caps in series and parallel.

[Chuck H]

my question is this: can I SAFELY run the line outputs of these amps in PARALLEL directly into the input of the third amplifier?

The real answer is, probably. The additional impedance added by most line out circuits should keep things safe. But the fact that all line outs are different (as you've observed) can cause imbalances or, because these circuits are often VERY low impedance right off the speaker output, you could even damage something. With two known identical circuits that have an impedance of several hundred ohms, at least, you should be fine. I wouldn't try it willy nilly with two unknown circuits though. as Enzo pointed out, it's generally a bad idea. Don't be surprised if you get ground loop hum.

can someone tell me why I see so many different pot and dropping resistor values for line out mods on the net? I've tried both a 5k (from a Soldano schem) and 1meg (Herzog) pot and can't really hear the difference.

Also covered by Enzo, with this kind of circuit it's more about the "ratio" than the actual value. I'm going to add that this is because the line out only needs to supply voltage, not current. Although the speaker output it runs off of IS designed to supply current. The impedance of whatever device you plug your line out into is likely to be high enough that almost no current is passing through the circuit. With no current there is no voltage drop across a resistance. So it hardly matters what value pot you use because it's just the ratio that determines the voltage and not the actual resistance.

But wait, there's more... I wouldn't use a 1M dropper on a Champ because the load will be too high. Too high an impedance has two disadvantages (and one tiny advantage) Long cables are more likely to load the signal (especially on the top end) and there is the risk of additional loading at the input amplifier since 1M is a common input impedance. You should always shoot for running a low impedance into a higher one to avoid these issues. The one tiny advantage to the higher impedance line out circuit is that there is less loading to the OT secondary. As far as that goes the difference between a 5k and 1M dropping resistance on a 4R or 8R load is really tiny enough to be ignored.

I use a 500 ohm pot for the load and a dropping resistor value chosen to create the right ratio for whatever wattage amp I'm building. For example, Your Champ is probably putting out about 4.5 volts at full tilt (not to be confused with watts as the line out is a voltage dependant circuit). If you are using a fixed line out you need to be sure the output doesn't exceed about 1 volt so you won't clip the input of any device you may plug it into (0dB (U or V) are about the most common input sensitivities and 1V is close enough) Since we've chosen to use a low impedance load to avoid loading at the input, lets say 470 ohms since it's nice and low and is a common value. Now we need to change 4.5 volts into 1 volt. 1 into 4.5 is 4.5, so we need roughly a 4:1 ratio (with the "1" being the load) to get our 1 volt output. Since this example is using a 470 ohm load, 4 times 470 is 1880. Since 1.8k is a common resistor value (and close enough) we need a 1.8k resistor to our line out followed by a 470 ohm resistor for the load.

But wait, there's more... Why choose a 470 ohm load? I've had trouble with highish impedance outputs run into effects (especially digital). Even without clipping I've heard crackling and cutting out. 600 ohms has been a common output impedance for a number of audio devices from early on so I think this standard has stuck and most input circuits are designed to be compatable with it. And did I mention the non loading advantage of a low output impedance?

Still more... I like to use a pot for the load (with the line out on the wiper) for better signal to noise ratio. It allows me to turn up the output when I want to run the amp clean. To use a pot I adjust the ratio for two volts output instead of one. That will give you the ability to turn up the line out voltage when the amp is only putting out enough to make .5 volts across a fixed line out. So to figure for a 500 ohm pot and a 2 volt output we 2 into 4.5 = 2.25. Roughly 2:1 500 times 2 is 1k. So you would use a 1k dropping resistor and a 500 ohm pot for the load with the line out on the pots wiper.

The circuit values in the above example are lower than what you've been been using so far but I promise that the total circuit impedance of 1.5k to 2.3k or so won't load your Champs output enough to care and it should perform better than the values you have now. Especially if you ever want to plug the Champ into an effects processor and then into a board or PA.

Line outs for amps are a very simple, common mod that it has always seemed to me are poorly understood. Not that I understand everything perfectly. But the way I've been doing it seems to address some issues that other line out circuits ignore.

Cheers

Chuck

[riz]

I have a cheap 4 channel mixer, and was thinking of trying it! Re: ohm's law-- yes I am aware of how fundamental it is, and it is obvious to me how important it is I burn it into my brain. I use it, I just usually refer to a book to make sure I have it right. Enzo, as you mentioned, I am beginning to see voltage dividers everywhere and I'm seeing schematics in a whole new light. I'm one of those read-the-book-from-the-back kind of guys. I cringe when I think back to some of the intuitive "field" repairs I pulled off on my old Marshall, out on the road, when I really had no idea what I was doing. I'm actually seriously considering taking an intro course in electronics at the local community college this fall... that is if I EVER finish re-working my woodshop! Having my functional knowledge of electronics catch up to my practical and historical knowledge (guitar amps specifically...) would be HUGE for me. I really appreciate the responses, and I'm sure I'll have more questions come Tuesday, which is the next day I will make time to go to my studio and tinker.

[riz]

Chuck, you rock. Thanks. We must have been typing at the same time. I had noticed when I run my prototype into (some) effects, something changes, and I've been sure it was loading. I will be digesting all this. Ya'll have given me what I was looking for and more.

[d95err]

Most "Herzog" style line out circuit designs I've seen use extremely high value resistances (like a 1M ohms). This is not at all necessary. I use something like a 4k7 pot and a similar value series resistor.

Fender used 2k2/220 ohms in this line out:
http://www.schematicheaven.com/fende...180w_schem.pdf

[Chuck H]

That isn't to say that those values make for a good line out. Because the ratio for the circuit should be based on an amps output wattage and impedance. This is one of the points I was making above.

Using your example above, if I installed a line out in my plexi Marshall half stack rig using a 4.7k dropper and a 4.7k pot the line out would be capable of delivering 20 volts!!! By contrast if you used the quoted Fender circuit of 2.2k dropper and a 220r load in a Champ the line out would only deliver about .45 volts with the amp cranked. Point is this isn't a stand alone type of circuit. The ratio needs to be designed for the amp it's going into.

I agree that lower resistances are better for this type of circuit. So long as you don't go low enough to interfere notably with the amps impedance load.

Chuck

[Enzo]

And that brings us back to the very common point that context is everything.

It ain't about the parts, it is about the circuit, and the circuit has to serve in the context.

[riz]

Thanks again for all the feedback!! It's reassuring to get advice that reinforces my hunches at this point in my education. I'm doing this mod for a friend, and I knew it would be a good project- it's forcing me to study! Today is the day to knock it out. My only hesitation is drilling holes in the chassis. It's a gigging amp, and a later silverface, so I'll be o.k. with it, it's just drilling that first hole... sure ya'll have been there. I have considered adding a project box inside the cabinet, but like I said, he gigs with it so it would be nice to have everything accessible, as well as it seems like a better job to contain everything within the chassis. Also I think we're gonna go ahead and add a vibro circuit to it. I have a '64 vibro-champ that sounds wonderful, and it looks to me that adding the circuit should be pretty easy. I would NEVER go beyond what I have already done to that amp (the '64...), which is caps, re-bias, and add a 1/4" speaker jack to it. I highly recommend the speaker jack change to any champ owners out there. Really adds to the versatility of this amp. (the original was shorting anyway, btw... now my Herzog "prototype" is merely a salvaged A/B box that plugs right in to the speaker jack--it has the power resistor, line out pot, bypass switch, and jacks for running it in a loop.) I'll let ya'll know how it goes...