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Post new topic Bass amp main and tweeter wiring question
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Author Topic:  Bass amp main and tweeter wiring question
Jerry Overstreet

Louisville Ky
Post Posted 28 Mar 2018 2:43 pm     Reply with quote

I'm doing a head/cab split on a Peavey TNT 115 combo bass amp. 4 ohm Blue Marvel 15 with a 3 1/2" horn, piezo I assume.

There are 3 speaker leads coming out of the amp. One for the 15 and one for the tweeter. They share a common ground.

I thought I should wire the new cabinet with a TRS jack and build a 3 contact speaker cable as well which I did.

The amp has an ext. speaker jack that is wired in parallel with the main. We are going to be using the 15 cabinet as a bottom and will be adding a 2x10 cabinet also with a tweeter, to use in the ext. jack. We want to keep the tweeter in the bottom cab for times when we want to use only the bottom 15 without the 2x10 cab.

My concern is this: This amp model has no provision for crossover and I'm wondering if the tweeter[s] likely has it's own built in crossover capacitor and whether it's really necessary to wire in a 3 contact TRS jack and speaker I making too much out of this?

Maybe just wire the tweeter in line with the 15 low frequency driver?

I'm wondering because the original amp speaker wiring has the 15 and the tweeter on separate leads.

I looked at the block diagram on Peavey's site, but it only shows the 15" driver with no reference to the tweeter, so I don't know the impedance or speaker load of the tweeter.

I don't find this exact driver anywhere, but I think this link is for a suitable similar replacement unit for this application.

I haven't been able to find any information concerning this particular situation on the bass amp forums or otherwise online.

Looking at some other head/combo ext. speakers etc. online, they don't seem to mention anything special about hook up.

Probably elementary stuff to some of you, but I've only built cabs for steel guitar use using one speaker for instrument use.

I'm familiar with dedicated 2 and 3 way crossover modules in home stereo systems, but that doesn't apply here.
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Georg Sørtun

Mandal, VA, Norway & Weeki Wachee, FL, USA
Post Posted 28 Mar 2018 5:09 pm     Reply with quote

Most would just wire the piezo tweeter in parallel with the woofer. Expect a harsh sound from the piezo.

Some would add a resistor of at least 5 Watt / 6 to 22 ohm, in series with the piezo to reduce harshness, balance levels of tweeter to woofer, and reduce the piezo's capacitive effect on the amp. (That's R1 in the rudementary scematics below.)

Some, like me, might take it a little further and tune the piezo's frequency curve and general loudness to match the woofer, by stacking RCs in parallel with R1. Cannot give you values for those, as I have no idea about the sensitivity and frequency response for neither woofer nor tweeter … I usually match them by listening and measuring the mounted units over the power- and frequency range they are supposed to cover.

Note: unless the woofer covers the frequency range up to about 3.5 KHz well, expect a "hole" between it and the piezo tweeter. Piezos like the one you linked to rarely ever go much below 4KHz.
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Jerry Overstreet

Louisville Ky
Post Posted 30 Mar 2018 11:25 am     Reply with quote

I'm not aware of any harshness of the tweeter while the amp was in it's combo configuration.

I was able to find something online concerning this issue. By reading that and Georg's reply, I'm guessing by what info I see concerning these piezos, they do have a built in crossover so they don't need a dedicated crossover module as found in FOH and Bi-ampable amplifiers, cabinets etc.

Also, since there is no mention of separate input wiring of the tweeter in satellite bass cabs that I found, that it isn't necessary.

My concern stemmed from finding two separate wires for the tweeter and LF driver coming from the combo amp.

Since I still don't really know and I have already wired the amp speaker jack out and the cab speaker jack in with TRS units, I'll leave those that way.

I'll assume there's probably an adapter or a cable that will sum the TRS jacks to just TS mono for use with other cabinets.

I've probably wasted time by wiring this all in and building a suitable equal stranded TRS speaker cable. Oh well, maybe this will help somebody else Oh Well

Still, he edits. OK, bear with me here. Looks like I'm talking to myself but, if the extension speaker jack is wired in parallel with a main speaker output jack wired trs, plugging a mono cable into it, the extension speaker jack, would cause a short because the ring contact of the jack would contact the sleeve [ground] of the mono plug since there is no isolation.

So what happens if we use a trs 3 conductor cable in the mono jack? Hmmm......
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Georg Sørtun

Mandal, VA, Norway & Weeki Wachee, FL, USA
Post Posted 30 Mar 2018 2:23 pm     Reply with quote

Jerry Overstreet wrote:
So what happens if we use a trs 3 conductor cable in the mono jack? Hmmm......
Ring won't get connected.

FWIW: piezoelectric tweeters don't have built in high-pass filters as such. They simply cannot reproduce low frequencies as sound – just as woofers cannot reproduce true highs – and burn off whatever lows they are fed as heat.

I used to build PA speakers – with piezo tweeters for high efficiency in the upper range – up until the mid -90s. Had no problems with mine, but had to replace piezos in other peoples setups quite regularly because they started sound awful and then quit working. Mine was given high-pass filters, while those I had to fix had none.
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