Adcom GTP-500 Preamp - Modifying "normal" output for subs

[jbeckva]

Ok, here's an idea that just popped into my head. I have the Polk speakers as the main drivers now, but I do sometimes miss the very bottom frequencies that the CV's provide. I would like to do several things here..

1. Use the CV's as "subs"
2. Without disconnecting the mids and tweeters on the CV's, drive them with a low pass preamp output (aka "subwoofer out" on some of them fancier preamp deals).
3. Use a separate amp for the CV's, since their sensitivity is much higher than the polks (106 versus 90). Level matching is much easier.

Now enter the Adcom GTP-500 I purchased recently. It has two sets of outputs - LAB and Normal. The LAB output is direct coupled - no coupling cap inline - while the Normal does have a cap inline.

I am thinking here.. instead of a simple cap coupling the "Normal" output, go ahead and modify it with a real (and probably pretty simple?) low-pass filter network. Take the crossover point of hmm.. say around 100hz (or wherever it "should" be i.e. right on an octave boundary somewhere.. right?).

First concern - how to do this without introducing any unintended side effects back into the Lab outputs. Should I use a unity-gain buffer, perhaps? OR.. hey... I think there are some example circuits with using op amps as both buffers AND low pass filters (a combination of sorts, eh?).

I think.. second concern.. PHASE.. As with anytime and depending on how you put a cap inline with a signal, isn't it going to phase shift it?

(dang.. time for some serious R n D here for me... BUT.. I think it'll be something cool to try out)

 

[Pure_Brew]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

An active crossover would require its own power supply, and the supply for the adcom is engineered for only what's in the chassis-no?

 

[speakerman1]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

What is the purpose of an active cross over?

Larry

 

[laatsch55]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

To cross over in a timely and purposeful fashion as opposed to being all passive about things.

Seriously, actve equalization takes place before final amplification, and does so with a power supply, thereby not attentuating the signal. Passive(as in, in cabinet crossovers) take ther energy of the signal and filters with the signals power, assuring a power loss.

Yep Jer, phase shifty is real, but with a well designed low pass that should be takencatre of. I have a book with a projectand plans for just such a thing. I'll dig it out when I get home tonight.

 

[speakerman1]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

To cross over in a timely and purposeful fashion as opposed to being all passive about things.

Seriously, actve equalization takes place before final amplification, and does so with a power supply, thereby not attentuating the signal. Passive(as in, in cabinet crossovers) take ther energy of the signal and filters with the signals power, assuring a power loss.

Yep Jer, phase shifty is real, but with a well designed low pass that should be takencatre of. I have a book with a projectand plans for just such a thing. I'll dig it out when I get home tonight.

See if I have this right. The x-over goes in before the amp? Power loss would be minimal depending on the x-over design. I have a DBX that is passive. Except for the lights. I'm one of these passive is good guys. I will read up on it.

Larry

 

[jbeckva]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

That's right, Larry. Since the pre has two sets of outputs, what I want to do with one is just put in a low pass filter .. then drive the CV's through a separate amp with that signal.

The pre's power supply itself will suffice, I think. It already has the standard +/- 15 needed for it's own buffers/op amps. I'm thinking that if I use an opamp with a low pass in it's feedback loop (or other standard design), that by the nature of it's very high input impedance it won't affect the signal that is split off before it, which will not be filtered and drive the Polks.

 

[speakerman1]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

That's right, Larry. Since the pre has two sets of outputs, what I want to do with one is just put in a low pass filter .. then drive the CV's through a separate amp with that signal.

The pre's power supply itself will suffice, I think. It already has the standard +/- 15 needed for it's own buffers/op amps. I'm thinking that if I use an opamp with a low pass in it's feedback loop (or other standard design), that by the nature of it's very high input impedance it won't affect the signal that is split off before it, which will not be filtered and drive the Polks.

Some of the actives has a sub output. I can understand the concept of not amplifying unneeded signals.

Larry

 

[jbeckva]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

Yeah but it's fun to experiment, and I have the spare parts laying around (I think?). And I'll be able to just use one box to accomplish my goal .. all of it should fit inside the preamp.

 

[speakerman1]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

Would you limit the freq. with the opamp? If not how then? Where would you pull your control from? So you will remove the low end cross overs out of the CVs. Or bypass them?

50 questions HUH. LOL

Larry

 

[jbeckva]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

Would you limit the freq. with the opamp? If not how then? Where would you pull your control from? So you will remove the low end cross overs out of the CVs. Or bypass them?

50 questions HUH. LOL

Larry

Yep, the opamp when you put a few caps here and there in the circuit turns into the filter. I'd keep the CV's as is probably - I could just disconnect the tweeters and midranges on it, but really with the filter in place before the amp, the amp will have more power dedicated to the low end and not be wasting it on the higher frequencies. I'll draw a pic of what I'm thinking in a few..

 

[speakerman1]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

I would just run 2 wires straight to the woofer from the speaker terminals. I have some silver wire you could twist to make the jumpers. You may be surprised at the sound difference.

Larry

 

[mlucitt]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

For a low pass LC filter you can choose a Butterworth design that has a relatively flat frequency response. For a 120 Hz cutoff, find or make a 15 mH inductor (it will be large) and a 220 uF 100 V non-polarized capacitor. At the output of the amplifier channel place the capacitor across the outputs (in parallel) and then form a "T" with the coil in line with the positive wire (in series).

You could also use a 4.7K 10 Watt resistor in series with a 47 nF 100V non-polarized capacitor in parallel to give you a 100 Hz low pass RC filter.

 

[jbeckva]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

For a low pass LC filter you can choose a Butterworth design that has a relatively flat frequency response. For a 120 Hz cutoff, find or make a 15 mH inductor (it will be large) and a 220 uF 100 V non-polarized capacitor. At the output of the amplifier channel place the capacitor across the outputs (in parallel) and then form a "T" with the coil in line with the positive wire (in series).

You could also use a 4.7K 10 Watt resistor in series with a 47 nF 100V non-polarized capacitor in parallel to give you a 100 Hz low pass RC filter.

Thanks, but I'm looking to filter from the preamp output to amp input. Reasoning is that I have two sets of preamp outputs one of which has a decoupling cap in series (easy removal and "insertion" of the filter in it's place, right?). Also if I filter it before the amp, then the amp itself won't be wasting power on the frequencies above the cutoff frequency of the filter.

I'm looking at something like this here.. which seems relatively simple to build.

[attachment=0:2aslsh5e]lpf.jpg[/attachment:2aslsh5e]

It's a first order, but I wonder if the added complexity of a 2nd order would be necessary. I'd like to use the opamp as part of the circuit as a buffer and to make sure there's no bleedover to the unfiltered pair of preamp outputs.

 

[laatsch55]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

Here ya go Jer, from G. Randy Sloan's " The Audiophiles Project Sourcebook"

 

[laatsch55]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

there is also a linear phased lo/hi pass filter in this book where a 4th order filter for both low and hi are identical in phase.

 

[mlucitt]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

You could also use a 4.7K 10 Watt resistor in series with a 47 nF 100V non-polarized capacitor in parallel to give you a 100 Hz low pass RC filter.

Thanks, but I'm looking to filter from the preamp output to amp input. Reasoning is that I have two sets of preamp outputs one of which has a decoupling cap in series (easy removal and "insertion" of the filter in it's place, right?). Also if I filter it before the amp, then the amp itself won't be wasting power on the frequencies above the cutoff frequency of the filter.

I think I basically recommended above what you described with the exception of the unity gain amplifier.

Mark

 

[jbeckva]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

Well sort of.. I plan on building it inside the preamp and using the same power supply. Since space is limited, I'll have to forgo any use of inductors of any meaningful value. The circuit I described is a first order with just a shunt cap for the HF. Do I need a higher slope than that, do you think? Do I need to watch out and make sure the Fc is low enough so that there's no rollover on the bottom end?

Maybe not tho, since the actual crossovers in the CV's will still be connected. I'd like to keep those intact so (in case of irritating neighbors with their bass boom box cars) I can always put 'em full range in back of a WOPL.

 

[jbeckva]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

How about this one here? Fc of around 80hz, which should give enough of the bottom end to round out nicely, maybe?

[attachment=0:hvo7molw]lpf2.jpg[/attachment:hvo7molw]

 

[speakerman1]

Re: Adcom GTP-500 Preamp - Modifying "normal" output for sub

What is the freq. specs on the woofer? I understand what you are trying to do. Just haven't figured out why. I'm going over about 200 scenarios in my head as to why. I like your idea better than an active.

Larry