Ampeg SVT3 Pro PREAMP OUT Balanced or Unbalanced? NEED A DEFINITETIVE ANSWER QUICK!!!

[agedhorse]

This. Try it and see for yourself. We did this all the time for returns down a snake into a pile of PA amps. Always worked fine even for that. It's not technically correct, but we never had a problem with it. Test it 'before' you use it at a gig. We were using a DBX160 as a line driver.

The reason this worked in this case is that the shields of the snake are all connected to ground via leakage within the snake. Normally, without the shield there would be noise build-up. It's always a compromise, but normally the shield must be grounded at one end (either through the sleeve of the 1/4" connector or a jumper between the grounded audio line and pin 1 on the XLR.

So I am going to ask a Sherlock Holmes question - what would be an approach to make the SVT3Pro into a 1000W amp (the number is generic)?

Buying a 1000W amp is an obvious answer but honestly, there is something about the SVT3Pro character that none of the other Pro series has. It comes the closest to the big CL and it comes close enough for my desires. Yet I would actually like to have the option of more headroom for some specific cases.

There isn't going to be an approach that's going to do what you are looking for, but you always have the option of micing the cabinet.

Are there no DIs that can safely work on an amp with a differential output? Is this a potential market opportunity?

I guess the limiting factor in this case is the SVT3-Pro output section probably requires a speaker load and being pushed to it's rails to develop the desired feel. So a speaker level DI really gets you nothing. Still seems like a lot of players want a speaker level DI that can be used safely with any amp.

It's an opportunity that brings little to the table other than grief. Too many variables and too many people who are willing to plug things together and try it without understanding what's really going on. I deal with this when helping players troubleshoot their problems and listen to them arguing how great the very idea that blew their amp up and telling me that "I just don't understand". Common sense ask how I can't possibly understand the products I designed, but common sense doesn't seem to matter as much these days

I hope this clears up the often misunderstood XLR/Balanced thing.

XLR and Balanced usually go together but are two different things. XLR is a three pin connector. Bal/Unbal is an electrical circuit. XLR can be balanced or unbalanced depended on the amp/preamp circuit but XLR is usually balanced. Just because there is an XLR connection doesn't guarantee it's a balanced circuit. I.E. You can plug an XLR to 1/4" adapter into a balanced XLR output and it then becomes "unbalanced circuit".

The difference between balanced and unbalanced is that the balanced method will provide noise rejection and a 6dB boost over unbalanced. That's it. The longer the cable run, the more likely you'll want noise rejection and the 6dB boost of a three wire balanced circuit.

This isn't the ultimate answer you were looking for but I hope it at least helps.

I'm sure you're aware that it's always a high risk to make a major change that hasn't been tested or rehearsed just before an important event. Taking that risk is your call.

Just as important, the ground/shield in a balanced circuit does not (and should not) carry any current. In a balanced circuit, if there is leakage current between the grounds, the shield conductor can be opened to eliminate this current flow.

 

[lokikallas]

Balanced VS unbalanced matter very little, as we are talking about a 3' jumper. The signal level is the only important factor, a matchbox, or DI might be the solution. That being said, using a 30 pound amp as a preamp seems all kinds of silly to me.

 

[ctmullins]

I agree with @Spectre Gunner , but I do want to clarify one comment:

I.E. You can plug an XLR to 1/4" adapter into a balanced XLR output and it then becomes "unbalanced circuit".

An XLR to TS (tip/sleeve) adapter will convert a balanced to an unbalanced signal, but a (properly wired) XLR to TRS (tip/ring/sleeve) will preserve the balanced signal. It’s a different physical format, but electrically the same.

Sorry for the pedantry, but it’s important to understand that not every 1/4” adapter is unbalanced.

 

[agedhorse]

Balanced VS unbalanced matter very little, as we are talking about a 3' jumper. The signal level is the only important factor, a matchbox, or DI might be the solution. That being said, using a 30 pound amp as a preamp seems all kinds of silly to me.

A DI isn't going to solve anything here, any level shifting that occurs will be in the wrong direction.

I agree with @Spectre Gunner , but I do want to clarify one comment:

An XLR to TS (tip/sleeve) adapter will convert a balanced to an unbalanced signal, but a (properly wired) XLR to TRS (tip/ring/sleeve) will preserve the balanced signal. It’s a different physical format, but electrically the same.

Sorry for the pedantry, but it’s important to understand that not every 1/4” adapter is unbalanced.

1/4" TRS can carry a balanced signal and is often used where there is no space to use an XLR connector. Unfortunately, they are often not well labeled.

1/4" TS are the connectors being discussed here, that amp only has an unbalanced line output.

 

[jeff62]

Fun watching y’all discuss and recommend the ideal solution when the OP left the thread a long time ago. He didn’t really want a workable answer, did he?

 

[agedhorse]

Fun watching y’all discuss and recommend the ideal solution when the OP left the thread a long time ago. He didn’t really want a workable answer, did he?

Of course he didn't, he appears to have known better all along and was looking for validation rather than a good answer.

Of course, this thread will remain valuable for those who refer to it down the road, there's some good general information presented.

 

[lokikallas]

So I am going to ask a Sherlock Holmes question - what would be an approach to make the SVT3Pro into a 1000W amp (the number is generic)?

Buying a 1000W amp is an obvious answer but honestly, there is something about the SVT3Pro character that none of the other Pro series has. It comes the closest to the big CL and it comes close enough for my desires. Yet I would actually like to have the option of more headroom for some specific cases.

Buy a 7 pro (rated at 1000w at 4 ohms) only 15 lbs, and one magical tube in the pre.

 

[ctmullins]

1/4" TS are the connectors being discussed here, that amp only has an unbalanced line output.

The block diagram shows that the Preamp Out is a TS 1/4”, but the Line Out is TRS, and it’s depicted as being wired parallel to the XLR, so I assumed that meant it was balanced? Sorry to niggle, I just want to educate myself as much as anything.

I’m looking at page 13 of this document: https://ampeg.com/pdf/SVT-3PRO_OM.pdf

 

[Nevada Pete]

The reason this worked in this case is that the shields of the snake are all connected to ground via leakage within the snake. Normally, without the shield there would be noise build-up. It's always a compromise, but normally the shield must be grounded at one end (either through the sleeve of the 1/4" connector or a jumper between the grounded audio line and pin 1 on the XLR.

There isn't going to be an approach that's going to do what you are looking for, but you always have the option f micing the cabinet.

It's an opportunity that brings little to the table other than grief. Too many variables and too many people who are willing to plug things together and try it without understanding what's really going on. I deal with this when helping players troubleshoot their problems and listen to them arguing how great the very idea that blew their amp up and telling me that "I just don't understand". Common sense ask how I can't possibly understand the products I designed, but common sense doesn't seem to matter as much these days

Just as important, the ground/shield in a balanced circuit does not (and should not) carry any current. In a balanced circuit, if there is leakage current between the grounds, the shield conductor can be opened to eliminate this current flow.

Thanks Andy. I asked Mark about this several times and he just said don't worry about, and it did seem to work. More desirable to do it right, though. Yes?

 

[Wasnex]

It's an opportunity that brings little to the table other than grief. Too many variables and too many people who are willing to plug things together and try it without understanding what's really going on. I deal with this when helping players troubleshoot their problems and listen to them arguing how great the very idea that blew their amp up and telling me that "I just don't understand". Common sense ask how I can't possibly understand the products I designed, but common sense doesn't seem to matter as much these days

I am not a fan of speaker level DI. 99.9999% of the time I would argue mic'ing the cab is a better option if you want to capture the mojo of the amp.

But there seems to be a lot of players who are going try and run speaker level DI, and nobody (that I am aware of) offers a product that makes the process safe with all amps. So a lot of people are going to continue to damage gear.

Since I don't personally like the sound of speaker level DIs, I am not framing this as a best audio practices. It's coming from the idea of responsible citizenry. In other words, since you can't educate or convince everyone of the hazard, what can be done to make the process safe?

Is it really a huge engineering hurdle to design a DI with a balanced speaker level input? My guess is it's not, but since no companies are offering this sort of product, perhaps there is no money to be made.

 

[agedhorse]

The block diagram shows that the Preamp Out is a TS 1/4”, but the Line Out is TRS, and it’s depicted as being wired parallel to the XLR, so I assumed that meant it was balanced? Sorry to niggle, I just want to educate myself as much as anything.

I’m looking at page 13 of this document: https://ampeg.com/pdf/SVT-3PRO_OM.pdf

Good point, I didn't consider this line output because it appears from the schematic that there is a 17.5:1 voltage transform at the DI output compared with the preamp out, which would put this level at a hot mic level. This is based on the test point data and a little math.

 

[ThisBass]

Ok, I have looked and searched everywhere I could think of and (unless I missed something) I cannot find a definitive answer. Yes I have looked at the manual and No it does not say. (Only mention is the DI)

The reason I'm needing to know is because I JUST ordered a Crown XTI 1002 (I plan on running in bridged mono) into a Schroeder 212pl 1200W rms using the SVT3PRO as a pre amp... I JUST realized that the CROWN has an imput impedence of 1.4v with BALANCED XLR inputs ONLY. *EDIT* Gig in TWO days! (Will receive on gig day!) Need to run my cab AND FOH using DI.

HELP.

The XLR-balanced out provides roughly microphone signal level which is too less of level to drive 1.4V input sensitivety of the crown.

You'll get better results with the unbalanced output cause it can drive higher voltage levels into the input of your crown amplifier. Of course you will need a proper cable that fits to 1/4" phone output into XLR input

 

[rickdog]

I guess the limiting factor in this case is the SVT3-Pro output section probably requires a speaker load and being pushed to it's rails to develop the desired feel.

The 3Pro output stage is... unique*. It has a tube driver stage (in a configuration that I believe is called a White cathode follower) that drives the gates of the output MOSFETs. The MOSFETs provide no voltage gain, just current gain. There's no AC feedback in the output stage (there's DC feedback to prevent DC offset at the output). There's a variable B+ supply to the driver tubes, controlled by the "Tube Gain" knob. This allows controlling the saturation characteristics of the driver.

So it has some character that is entirely independent of the load on the output.

However, in my experience with my 3Pro, the magic is in the preamp, not the power amp (frankly, I don't like the "tube gain" saturation characteristics). So I think the OP's idea of providing massive clean power to reproduce the preamp signal accurately isn't so crazy.

* There's a schematic for the 3Pro, or at least a link to it, floating around here somewhere. If you can't find it, I'll dig up my copy.

 

[ctmullins]

I have no idea what you just said.

 

[Wasnex]

The 3Pro output stage is... unique*. It has a tube driver stage (in a configuration that I believe is called a White cathode follower) that drives the gates of the output MOSFETs. The MOSFETs provide no voltage gain, just current gain. There's no AC feedback in the output stage (there's DC feedback to prevent DC offset at the output). There's a variable B+ supply to the driver tubes, controlled by the "Tube Gain" knob. This allows controlling the saturation characteristics of the driver.

So it has some character that is entirely independent of the load on the output.

However, in my experience with my 3Pro, the magic is in the preamp, not the power amp (frankly, I don't like the "tube gain" saturation characteristics). So I think the OP's idea of providing massive clean power to reproduce the preamp signal accurately isn't so crazy.

* There's a schematic for the 3Pro, or at least a link to it, floating around here somewhere. If you can't find it, I'll dig up my copy.

I believe there are some instances where people drool about the tubey deliciousness of their output tubes, but in reality, it's the driver tubes reaching saturation. Perhaps the Mesa D180 is an example.

I have never played a 3-Pro, so I have no idea where the magic comes from . It's possible some people like the magic of the preamp and others find the magic elsewhere. I think using a massive power amp would work very well for some, but it probably would not satisfy everyone; especially those who think there is magic in the 3-Pro's output section. Tubey deliciousness can be quite a confounding subject .

 

[agedhorse]

While the preamp output is stated as .7V nominal, I think it can in fact deliver quite a bit more than that before distorting.

 

[BassYerFaceoff]

I predict damages speakers in your future unless you are very careful. An "accident" will destroy your speakers very quickly at that power level.

According to their website, the cabinet is 800 watts RMS, and is available in both a 4 and 8 ohm
version. Do you know which version you have? This may be more important than your preamp being balanced or unbalanced.

Sorry everybody, just had the chance to log back on and see the messages. I also want to appologise for coming across as an "A-hole" last night... just stressed.

To @agedhorse... I got a CUSTOM built 1200w RMS @4ohms 212pl (2400w peak) and planned on using the Crowns limiters to reign in any damage...I also ALWAYS admire your input on threads!

To everyone else... that's as far as I've made it looking at the replies... so gimme some time. lol bbiab And THANK YOU ALL for chiming in!
*EDIT* 103db 1w/1m sensitivity

 

[BassYerFaceoff]

From Ampeg’s website:

For amplifier support or any questions concerning owner’s manuals or schematics please open a ticket here or call 818-575-3600 option 2

Yeaahhhh... haha what A joke calling them was. They don't know. Ampeg is now owned by Yamaha (happened early this year.. like May 2018)... The "TECH" there didn't know.

 

[agedhorse]

I'm not aware of ANY 12" driver with voicing suitable for bass that's going to hold up at 600 watts RMS. Custom built or otherwise. That's why I made my suggestion.

 

[BassYerFaceoff]

He's using custom built B&C drivers in these... there are some that will hold up to even more!