Need High-Pass Filter Mod

[tbryanh]

How can V1A be converted into a high-pass filter that rolls off at 24 dB per octave at about 70hz?

[diagrammatiks]

you can try replacing the cathode bypass capacitor with a 1.5uf

[loudthud]

24dB per octave is gonna take at least two op-amps, three if you want to stabilize the source impedance.

[tbryanh]

If the circuit cannot be converted with passive components, I am willing to use an external active crossover unit. Probably a rack-mount device.

It seems rack-mount devices are for line-level signals and might not work with instrument-level signals. I have heard that line-level devices have low input impedances (10k instead of 1m). Don't know if the lower signal level is an issue. Might be only an input impedance issue.

The feed is coming from a mixer. The mixer can output either microphone-level or line-level signals.

Maybe the mixer output, when set to microphone-level, can feed a low-impedance line-level rack-mount device without any problems.

[tbryanh]

Using a rack-mount device is only a second choice.

The first choice is to add inductors and capacitors to V1A.

[tbryanh]

If the circuit cannot be converted ... a rack-mount device....

A module, if available, would probably would probably be better than a rack-mount device.

[tbryanh]

Elliott Sound Products has a 24 dB/Octave 2/3-Way Linkwitz-Riley Electronic Crossover. Linkwitz-Riley Electronic Crossover

Here is something by Linkwitz Lab. http://www.linkwitzlab.com/filters.htm#3

These are diy modules. I prefer ready made modules.

[J M Fahey]

*Very* short answer: you can't.
Not even with a lot of inductors or capacitors.
The sensible answer is Louthud's, so now that you know the truth, please reformulate your priorities.
Some probably working options:
1) Since you can come with an already amplified signal, use the line level one, which will match a rack-type crossover very well, and *after* going through it, pad it down to guitar level, to match it to your Tube Amp input
*or*
2) If you want to keep it *inside* the Tube Amp, plugging your Guitar straight into it, build a PCB containing what you need.
Basically it would be what Loudthud suggested: a 1M input impedance Op Amp stage, 2 x 12db/oct stages @ 70 Hz (1 or 2 Op Amps each).
The input stage might have a moderate gain (2 to 4 x) and later you'd pad the output by the same amount, for a slightly better noise performance.
3) just a personal curiosity: what problem are you trying to solve?

[tbryanh]

1) Sounds good.
2) Don't want to do any diy right now
3) I want to run a bass amp in parallel with a guitar amp. Need to protect the guitar speakers.

[Enzo]

Well then, put a passive crossover between amp and speaker. That won't require adding op amp power supplies to your tube amp, or for that matter anything inside your tube amp.

And why does it have to be a brick wall 24db/octave? Start rolling off at say 90Hz with a gentler slope. You won't miss the decibel or two between 70 and 90.

[tbryanh]

... put a passive crossover between amp and speaker....why does it have to be a brick wall 24db/octave? Start rolling off at say 90Hz with a gentler slope. You won't miss the decibel or two between 70 and 90.

I guess my thinking was locked into hifi and pro audio standards after participating in this thread: http://music-electronics-forum.com/t25652/

By strict hifi and pro audio standards, no two drivers can share or overlap their frequency bands. This is why they want steep roll offs.

I am trying to blend two amps, so a steep roll off would probable be wrong. The amps will blend better with a gentle slope. A passive crossover at 90Hz or so should be fine.

I will check this website out for help in designing a crossover: Shavano Music Online - Do It Yourself Construction Articles

Thanks

[tbryanh]

Since bass has the most energy, to preserve the headroom in the guitar amp, it might be best to roll off at the input of the guitar amp instead of at the speakers.

I could probably use some kind of stomp box EQ pedal device in front of the guitar amp. This sounds like the simplest solution provided a pedal exists.

I am still open to modifying the V1A stage. Since the roll off is going to be gentle, it might be doable.

[J M Fahey]

OK, nowthat the game rules are on the table, I can suggest a simple minimalistic solution.
We can add 3 *very* simple 6dB/oct filters, which combined will be quite close to a "real" 18dB/oct filter.
Referring to your schematic:
1) Connect V1Pin1 straight to the right side of 1M grid resistor.
Open the link that joins both 33 K input resistors to that point, and insert a 2200pF capacitor replacing it.
Presto!!: a first 6dB/oct @ around 80 Hz.
2) Replace the 750uF cathode cap by a 1.5uF one (1uF//.47uF ; // meaning: "in parallel").
Now we have a second around 80Hz highpass.
3) Cut the link between the 250 K Treble pot cursor and the 1M Volume pot "Hot" leg, and insert another 2200pF capacitor, as you did on the input.
You now have a 3rd 6dB/oct, also around 80Hz.
Don't worry about 1% precision parts, original Moby Dick whale oil and Ramses VII papyrus fillings and the like, just get regular good quality ones.
4) Rather than an exact crossover frequency or slope, the real problem is that steep crossovers, this simple or as complex as you wish, introduce important phase shifts around the crossover frequency (no, not 180º, that would be easy to correct), which will cause funky cancellations at certain frequencies.
I suggest you do a final system tuning by ear, moving the guitar speakers forward and backwards (up to a few feet) listening for the best sound.
Good luck and post a few pictures of your rig.
PD: next time you post a schematic, please add designators (such as R1, C3, etc.) to relevant parts, to avoid referring to them as "the one slightly on top and somewhat to the right of ____ but not the one that ____)"

[tbryanh]

Wow. Sounds cool. Thanks.

[Enzo]

There you go.

I would quibble at strict standards demanding no shared freqs. It is done all the time. The standard is whatever results in a smooth overall response from the system. A lot of simple speaker systems run the woofers full range, and cross in the tweeters to add presence and definition at the top.

If you do it Juan's way, then you need no speaker crossover. But if you ever need to make a speaker crossover, you can design them from scratch easily enough, but places like Parts Express, sell them made up, plus they also sell bare boards ready to populate with parts. They sell the parts too, and offer charts for frequency so you can just look up values for your target freq. They come in simple or multistage for sharper rolloffs. As well as two way or three way.

And bass may have more energy, but it has to move a hell of a lot more air with it. I maintain that 10 watts of screaming 3k from a tweeter seems an awful lot louder than 100 watts of 100Hz. COnsider a 100 watt guitar amp is generally pretty darn loud, while a 100 watt bass amp is no earth shaker.