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Post by cynical1 on May 8, 2012 14:08:26 GMT -5
Just tossing this out for a quick test of the waters. I've been looking at the Fender Super Champ XD "Vintage Modified" amp, but would love to have an effects loop on the amp...it doesn't come with one by default. I know it has one preamp and two power amp tubes, and I'm not sure how that complicates the issue. I read something about dropping an opamp or another tube between the existing stages, but don't know enough about tube amps to make much sense of any of it. Here's the schematic: support.fender.com/schematics/guitar_amplifiers/Super-Champ_XD_schematic.pdfAny thoughts, warnings or just WTF are you thinking offerings are appreciated. HTC1 |
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Post by JohnH on May 8, 2012 15:32:23 GMT -5
My unqualified thought, would be to put send and return jacks in the wire that goes from the last opamp to the first tube stage (via the 0.1 cap and 15k resistor). At that point, its after all the digital effects and you can feed into the old-skool tube stage, but without needing to actually break into it and you are still dealling with low voltages. The first test would be just to wire a send, and check that putting that into a stomp box is giving a suitable level for the pedal to deal with.
John
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Post by sumgai on May 9, 2012 0:02:32 GMT -5
c1, The first test would be just to wire a send, and check that putting that into a stomp box is giving a suitable level for the pedal to deal with. You already have a Send, it's labeled as Line Out. All we need now is to insert a Return, with suitable amplification (one stage will do it), and a Level Control, and you're good to go. That's where the Return line will enter the fray. Once again, my meds are interferring with my ability to intuit someone else's intentions.... John, I could take that clause as 'break into the inside of the amp' or as 'break into the signal line' or..... what's your intention here, please. cyn, you do realize that there are no less than four (4) stages of solid state pre-amplification, before the signal gets to the first tube, right? And that tube, or more correctly, half of that dual-triode, is merely a buffer, not much in the way of amplification going on there. The other half is the Phase Inverter (PI), which makes the two output tubes work nicely together with the output transformer. All in all, not a bad circuit, but it's certainly not the Champ of yesteryear, (Nor for that matter is it the Super Champ that was designed for Fender by P. Rivera.) Should we presume that you have a line on one of these buggers, at a more-than-friendly price?  sumgai |
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Post by reTrEaD on May 9, 2012 2:57:48 GMT -5
And that tube, or more correctly, half of that dual-triode, is merely a buffer, not much in the way of amplification going on there. The other half is the Phase Inverter (PI), which makes the two output tubes work nicely together with the output transformer. I'm half in agreement with you here. The second half is a cathodyne phase splitter. No gain there. But the first half is the same sort of resistance coupled amp you'll see throughout Fender preamp stages. The minor difference is the 47 ohm resistor to accommodate NFB. Other than that's we see standard "Fender" component values for the cathode and plate resistors and the bypass cap. Looks like an appreciable amount of gain there. What is worth noting: The size of the grid resistor (10k) and the series resistor leading to it (15k). The modest attenuation at this divider 1/2.5 doesn't trouble me. But the load this would place on the line in (25k) should be noted. The output of most effects units should have no problem with this kind of load. But if the output cap in the effects unit is too small, expect a loss of bass. You already have a Send, it's labeled as Line Out. I'm not saying using the Line Out as a send is necessarily a bad idea. But I'm confused on what's happening with that part of the circuit. We have a nice little zero gain buffer with a 1k resistor feeding the tip of the jack. Looks great. The resistor will protect the opamp if there is an accidental short to ground in the outside world. But what's all that extra shiz connected to the ring of the jack for? And there seems to be a FOURTH lug, just below the sleeve. What's the purpose of that? Personally I get a little uncomfortable tying into something I don't fully understand. |
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Post by JohnH on May 9, 2012 4:14:43 GMT -5
You already have a Send, it's labeled as Line Out. sumgai That line-out does not look to be tonaly neutral to me. Most probably it has some ' cab simulation' within its filters and loops, so not what you necessarily want to always feed the power amp with. J |
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Post by reTrEaD on May 9, 2012 10:00:54 GMT -5
^ C16 providing positive feedback? Holy crap. I looked right past that the first time around.
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Post by sumgai on May 9, 2012 13:04:52 GMT -5
By definition, Line Out contains no tone-shaping circuitry, in and of itself.
All that whiz-bang is merely so much over-engineering..... C2, C35 and R90 do indeed shape the tone somewhat.... at a frequency well above the normal audio band. I'd be willing to wager that they, along with C14 and C16 are all meant to quell parasitic oscillations. Long time designers and experimenters will recognize these techniques as band-aids for poorly behaved IC designs (read: first or second generation components, made before SPICE-aided design was available).
Like upside-down reTrEaD, I've never before seen a "switching barrel" jack used in audio/professional music gear. This bears further investigation, if for no other reason than curiosity.....
sumgai
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Post by reTrEaD on May 9, 2012 16:22:11 GMT -5
I was racking my brain trying to figure out how C16 could work as a bandwidth limiter to reduce parasitics caused by phase margin. It's in the wrong place. Instead of being in the negative feedback loop, it provides positive feedback. Then it his me like a ton of bricks. This isn't about parasitics. This is Sallen Key topology. I should KNOW this.
C14, C16, R29, R31 and U2-A are a unity-gain second-order low-pass filter.
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Post by sumgai on May 9, 2012 21:54:37 GMT -5
^^^ And thus the torch is passed.
The pebble is no longer in the palm of my hand, it is now in reTrEaD's hand!
Which means I can now retire to the sidelines in peace, knowing that The NutzHouse is in good hands. ;D
Don't worry, I'm not leaving, but I'll certainly be putting my oar in the water less often, now that we have another fully qualified techster on board. Trust me, this is a good thing!
sumgai
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Post by reTrEaD on May 10, 2012 10:17:40 GMT -5
^^^ And thus the torch is passed. You might wanna keep one hand on it. I tend to be clumsy. Dropping a torch seems like a fire hazard. |
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Post by cynical1 on May 10, 2012 10:50:50 GMT -5
^^^ And thus the torch is passed. You might wanna keep one hand on it. I tend to be clumsy. Dropping a torch seems like a fire hazard. Please don't be dropping any torches on my amp...I just got the damn thing. So, to answer SG's question, yes, I had a very good line on one of these, and I now have one of these. It really is an impressive little piece of equipment, especially for what I paid for it... But I digress... It looks to me, a rank amateur, that dropping an effect loop in just prior to what looks like C64 and R25 is my best bet. Again, this is just a guess based on my limited knowledge and some time on Google. As I understand it, putting a pot in the circuit to act as a pan\fader would allow me to manually bypass the loop for Channel 2 (the one with all the simulations in it) as well as select it for Channel 1. If I understand what I've read so far, I will be looking at a 3dB drop with the traditional pot & 4 resistor pan circuits. Have I got this right? I really only see the point in using the loop in the "clean" or non-simulated channel, as the simulations are actually quite good. The effects loop is really only for the modulation effects, as I'd still run the overdrive and distortion pedal in the front door. Granted, it's a solid state preamp, from what I can divine, but it makes switching from one amp to the other simpler if I keep this sequence intact. Please feel free to shout out a solid WTF if I've got things seriously shanked. Happy Trails Cynical One |
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Post by reTrEaD on May 10, 2012 18:53:16 GMT -5
It looks to me, a rank amateur, that dropping an effect loop in just prior to what looks like C64 and R25 is my best bet. Again, this is just a guess based on my limited knowledge and some time on Google. That's what John suggested and I concur. In essence you'll break the connection on the right side of C64. I'd suggest adding a 1k resistor in series with the signal before it leaves the SCXD. That way if something shorts in a cable or pedal, you won't damage the output of U2-A. As I understand it, putting a pot in the circuit to act as a pan\fader would allow me to manually bypass the loop for Channel 2 (the one with all the simulations in it) as well as select it for Channel 1. If I understand what I've read so far, I will be looking at a 3dB drop with the traditional pot & 4 resistor pan circuits. Have I got this right? Dunno about that. I guess is all the voicings and effects are done by a module plugs into that multi-pin connector at location D5. (use the letter for the vertical and the number for the horizontal) If you saw something about installing a pan/fader on the net, drop a link? |
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Post by sumgai on May 11, 2012 1:10:04 GMT -5
cyn, Essentially, the job of mixing the Dry and Effect(ed) signal is best left to whatever devices(s) you insert into the Loop signal path. The reason for that is, you don't want to have the in-amp Dry signal going to the power amp alongside of the Loop's Return jack signal - what if they're out of phase? Although you, as an individual, could add a phase-inversion switch, most amps don't have such.... cost, time/effort to install it, panel space, etc. Your call, of course.  HTH sumgai |
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