VC508_Line_Out_Without_Speaker_Kill

[hewo]

Please assist circuitry reconfiguration of subject single ended amplifier, or is request not achieveable?
The church needs the line out for their ambience PA but i need the VC508 combo's 8" paper speaker onstage ear level (propped up).
I checked the schematic but it looks like energies (paper_speaker or line_out) are vastly different - boogie takes line_out from the enormous speaker energy. But VC508 is different.

[Chuck H]

The typical speaker driven line out can be designed for any amp. You just need to know your wattage and your load impedance. The line out would be a voltage divider designed to achieve about 1V from the AC at the speaker.

[Jazz P Bass]

If you want a plug & play answer I would recommend a 'speaker emulator' DI box.
Behringer Ultra-G GI100
Hughes & Kettner Red Box Pro

[hewo]

VC508_Line_Out_Cont'd

VC508.pdf

The typical speaker driven line out can be designed for any amp. You just need to know your wattage and your load impedance. The line out would be a voltage divider designed to achieve about 1V from the AC at the speaker.

Thankyou Chuck,
1. can u pl draw a picture how to parasytically divert speaker energy into the 10K? 100K? log taper potentiometer which feeds the line out jack receptacle?
2. parasytic diversion of spkr energy appears simple to implement.
3. but how difficult is it to implement NE5532 dual opamp (parasytic energy derived from full bridge rectification of 6.3vac heaters for plus minus rails of converted DC, trivial milliamperes consumed, 5-10ma each rail)?

[Jazz P Bass]

Marshall has done this for years.
They call it DI. (direct inject)
A 2.2K, a 100 & a 560 ohm resistor right from the secondary of the output transformer.

[hewo]

Thankyou Jazz Precision Bass for your responses

Marshall has done this for years.
They call it DI. (direct inject)
A 2.2K, a 100 & a 560 ohm resistor right from the secondary of the output transformer.

I looked at the schematic but my chinese 5F1 kit does not have the multiple spkr taps which marshall robs lineout energy from the 8 ohm windings. the chinese kit's chrome ot (so puny in size compared to the huge chinese PT) contains only one spkr winding, 4 ohms. i don't know whether an ot upgrade is adviseable being that the legacy circuitry did work spectacularly. why did designers of the 5F1 select 4 ohms? why not 8ohms? was there benefit to be had by their selection of 4 ohm? what of spkr's during that era? did they have 16ohms then? or factory was incapable (immature fabrication technology) producing 16ohms?

[hewo]

sorry folks but the post theme involves the VC508 LineOut but please do consider LineOut for my chinese 5F1 kit which does not have LineOut

[Steve A.]

I looked at the schematic but my chinese 5F1 kit does not have the multiple spkr taps which marshall robs lineout energy from the 8 ohm windings. the chinese kit's chrome ot (so puny in size compared to the huge chinese PT) contains only one spkr winding, 4 ohms.

I would think that the Marshall circuit should work fine with the 4 ohm output of your OT with some tweaking. The signal level from a 5 watt amp might probably be low compared to the 50 or 100 watt Marshall amps so you might want to do some tweaking anyway. Of course you might want to change the OT anyway as an upgrade.

Steve Ahola

P.S. Here is a circuit that Bruce Collins of Mission Amps drew up in 1998 for a volume reducer/direct box for small amps up to 25W.

[hewo]

Thankyou Steve A for your responses

I would think that the Marshall circuit should work fine with the 4 ohm output of your OT with some tweaking. The signal level from a 5 watt amp might probably be low compared to the 50 or 100 watt Marshall amps so you might want to do some tweaking anyway. Of course you might want to change the OT anyway as an upgrade.

Steve Ahola

P.S. Here is a circuit that Bruce Collins of Mission Amps drew up in 1998 for a volume reducer/direct box for small amps up to 25W.

[ATTACH=CONFIG]20245[/ATTACH]

1. I am confused whether to steal the lineout signal prior 6V6GT amplification or post 6V6GT amplification
2. Catching it at the spkr appears prudent to claim the whole amplification circuitry
3. But maybe it's more stable and easier to dial in on a line out level control pot which infiltrates circuitry actively, say NE5532, say TL072, because schematic shows availability of DC rails, and these modern opamps burn mininscule energy. In theory, a simple jfet, 2sk170, J201, etc., could do the job by buffering the low impedance spkr from the line out control pot, pot being needed to match sound engineer's conditions

[Chuck H]

hewo!!! Stop it! Your complicating the matter. Do you want a speaker driven line out? There are no additional amplifiers needed for that. The formula is easy. See the chart below for your starting voltage.

Make a voltage divider to achieve about 2V. So if your starting with a 5F1 we have five watts at four ohms. That gives us a starting voltage of 4.47V. Actually, 5F1's don't make five watts in reality. So lets just use 4V as the starting voltage and that makes the math easier anyway. We need 2V at the end of the voltage divider. Other criteria would be a reasonably low output impedance for the line out so that it can run some intrument cable. Also, we need a reasonably high impedance for the firs resistor in he divider to be certain that the circuit won't draw woo much current off the actual speaker load. So... Many outputs for analog gear is about 500R. Our load resistance will be a 500R pot. We need to cut the voltage in half. So the leading resisor will also be 500R (actually 470R since that's a common value and close enough) Since tha's over two hundred times greater load that your speaker there should be no significant current through the line out circuit. So, for the 5F1, A 470R resistor and a 500R pot make the divider and your line out provides an output of 0 to 2VAC. Why 2V? Because you may want to run the amp clean and still have enough juice from the line out to feed a PA, recording device or some other thing that likes line level for the lowest noise floor. And 2VAC isn't so much as to cause major problems if it's up too high.

The speaker is a current driven device. The line out is a voltage driven device.

See chart below, just use a 500R pot for a load resistance and you can figure a leading resistance for any amp. If the amp has multiple impedances just put the line out circuit on a specific secondary rather than the output jack.

[Steve A.]

Just wondering if you have talked about your plan to the soundman at your church- some or most of them would not want to connect a homemade device to their PA system. At least that was my own experience.

There are all sorts of DI boxes but for a 5F1 I think it that you would get the best signal just miking the cabinet. Is that an option?

Steve Ahola

P.S. Chuck- I got a stiff neck trying to read your chart so I took the liberty of rotating it 90 degrees clockwise...

watts-volts-ohms-chart.pdf

I hadn't heard that quote from R.G.- it makes a great signature!

"I have George Washington's own axe, the one he chopped the cherry tree down with. 'Course, it's been repaired over the last couple of centuries; it's had three new handles and two new heads." R.G.

[hewo]

Thankyou Chuck H and Steve A for your responses

hewo!!! Stop it! Your complicating the matter. Do you want a speaker driven line out? There are no additional amplifiers needed for that. The formula is easy. See the chart below for your starting voltage.

Make a voltage divider to achieve about 2V. So if your starting with a 5F1 we have five watts at four ohms. That gives us a starting voltage of 4.47V. Actually, 5F1's don't make five watts in reality. So lets just use 4V as the starting voltage and that makes the math easier anyway. We need 2V at the end of the voltage divider. Other criteria would be a reasonably low output impedance for the line out so that it can run some intrument cable. Also, we need a reasonably high impedance for the firs resistor in he divider to be certain that the circuit won't draw woo much current off the actual speaker load. So... Many outputs for analog gear is about 500R. Our load resistance will be a 500R pot. We need to cut the voltage in half. So the leading resisor will also be 500R (actually 470R since that's a common value and close enough) Since tha's over two hundred times greater load that your speaker there should be no significant current through the line out circuit. So, for the 5F1, A 470R resistor and a 500R pot make the divider and your line out provides an output of 0 to 2VAC. Why 2V? Because you may want to run the amp clean and still have enough juice from the line out to feed a PA, recording device or some other thing that likes line level for the lowest noise floor. And 2VAC isn't so much as to cause major problems if it's up too high.

The speaker is a current driven device. The line out is a voltage driven device.

See chart below, just use a 500R pot for a load resistance and you can figure a leading resistance for any amp. If the amp has multiple impedances just put the line out circuit on a specific secondary rather than the output jack.

Line Out for VC508 and Chinese Kit 5F1:
1. Okay, I understand what Chuck H is suggesting: take the hot spkr node and tie it through two resistors in series to earth, then tie the potentiometer high leg to the node created between the aforementioned two resistors in series. the pot's wiper is then tied to the hot terminal of the line out receptacle. the low leg of the pot is not connected to anything
2. Problem: when the preamp's 1M log pot is dialed low at about 9am, and, the power amp's 1M log pot (MV replaces the 6V6GT grid tiedown 220K fixed resistor) is also dialed in low, say, 11am, the hot spkr node will see much smaller voltage, and in turn, the line out (log too?) pot will have to be throttled wide open to obtain targeted 1Volt rms for line level output. ViceVersa, when both, the preamp and poweramp pot's are dialed higher, say 12pm and 12pm, the spkr node voltage gets BIGGER, and the line out pot must be throttled back smaller to achieve 1V rms. There's no AC rms meter telling the guitarist the line out pot has been dialed in to the targeted 1VAC rms. what's happening here is we're trying to span the whole range of hot spkr node voltages which will vary according to the gain/master volumes settings, by selecting the proper line out resistors/pot values.
3. If I hunt down an isolation impedance matching transformer (small inter-stage maybe?), the high impedance side could be piggybacked onto the hot spkr node, and the low impedance windings could drive the line out pot and dwnstrm line out receptacle. I would think the high impedance windings would not "bear down" onto the OT or the spkr. Is such an isolation transformer available? it seems pretty easy to do this.

[Steve A.]

hewo:

You said that you wanted a line out for when you are playing in church- wouldn't the level of your amp basically stay the same since you are just using it for a monitor? I would go with Chuck's recommendations.

The reason that most people add a master volume to an amp is so that they can crank up the preamp until it distorts, but at a softer volume level featuring preamp distortion rather than power amp distortion. With a 5F1 everything contributes to the distorted sound when cranked up: the preamp tube, the 6V6, the OT, the speaker and the cabinet. Here are two threads I found when googling "master volume 5F1." The first one illustrates the general consensus on adding a MV to a 5F1- there is one post that recommends using a VVR to lower the output volume. The second thread discusses replacing the 220k resistor with a 250k resistor and adding a 1k5 grid stopper.

Fender 5F1 with master vol ? - Seymour Duncan User Group Forums

AX84.com - The Cooperative Tube Guitar Amp Project

Steve Ahola

[Steve A.]

hewo:

You said that you wanted a line out for when you are playing in church- wouldn't the level of your amp basically stay the same since you are just using it for a monitor? I would go with Chuck's recommendations.

The reason that most people add a master volume to an amp is so that they can crank up the preamp until it distorts, but at a softer volume level featuring preamp distortion rather than power amp distortion. With a 5F1 everything contributes to the distorted sound when cranked up: the preamp tube, the 6V6, the OT, the speaker and the cabinet. Here are two threads I found when googling "master volume 5F1." The first one illustrates the general consensus on adding a MV to a 5F1- there is one post that recommends using a VVR to lower the output volume. The second thread discusses replacing the 220k resistor with a 250k resistor and adding a 1k5 grid stopper.

Fender 5F1 with master vol ? - Seymour Duncan User Group Forums

AX84.com - The Cooperative Tube Guitar Amp Project

Steve Ahola

From the SD thread: "I made a 5F1 circuit and you do have a master volume. Its called your guitars volume."
Then again it might be more accurate to say that the guitar volume control is Gain and amp volume control is the MV.

[Enzo]

ANY line out that derives its signal from the speaker output will have its level go up and down with the amp volume. If i turn my amp down I expect my signal level to go down. Setting levels with the PA or recording deck are what SOUND CHECKS are for. If you replace the resistor network with a transformer (far more expensive) you STILL have that problem. Can I turn up my pre gain and my master volunme down, yes, but what matter to the speaker driven line out is how loud the speaker is playing, it doesn;t matter how the volume level got there, it is none the less there.

If I play at some volume all week, then next week I play at lower volume, it is the sound mixer's job to adjust his levels. We don;t want a line out that always make the same level, you woould lose your dynamics. This is no different from using a microphone in front of the speaker, if you play quieter, you get lower signal levels. That is why mixers have level controls.