fdecks HPF, Part II

[MarkA]

Question on a possible different use of the fdeck:

I picked up a little tube pre -- a simple, single 12AX7 affair with one, unbalanced output. Getting all kinds of 60-cycle hum with it. This might or might not have anything to do with an impedance mismatch, but the maker of the pre says that it needs to feed into an amp with a higher input impedance than its output impedance. Output impedance of the pre is about 40 KOhms. Input specs on my two poweramps are 20 KOhms for the one (an old Lab Gruppen fp2200) and 7.5 KOhms on the other (a Crest ProLite 2.0).

So, I'm wondering if feeding the pre into the fDeck, then the fDeck into the poweramp might work to rectify that. I have an HPF3. It's unity gain. Signal levels would be way hotter than anything coming from my pickups -- the Crest needs .75 V or so, and the tube pre will deliver more than that. I don't want to harm my HPF!

The other possibility might be trying one of those little passive transformers folks use inline with a mic to feed it into an instrument input. I think we have one of those lying around from my cassette tape 4-track days!

ADDENDUM:
Just checked some specs on the HPF-Pre homepage. Input range for Series 1 and 2 is listed as "1 V p-p with "musically forgiving" overload response". For Series 3 (which I have), "6 V p-p with hard clipping response". Maybe it would be okay with the tube pre, then? As I said, I know that it will put out way more than the .75 V the Pro Lite needs (and the Lab can take more than that), but 6V seems like it would be a safe margin. Don't know if the HPF's 10 MegOhm input impedance is what I need or not, and don't know anything about the output impedance.

Any input is appreciated.

Thanks,
Mark

 

[MarkA]

Okay, to answer my own question (somewhat): I tried the HPF3 between the pre and the Lab Poweramp (20 KOhm input impedance) and it did make a marked, positive difference -- it opened up the top end a bit, made the sound more clear and balanced, and opened up the dynamics quite a bit -- but it did not eliminate the 60 Hz hum. The feel went from being compressed and mushy to hard and somewhat brittle (still, yes, this was an improvement), so perhaps the 10 MegOhm input impedance of the HPF3 is higher than ideal for this purpose? Maybe 1 MegOhm or thereabouts would be better? My knowledge is limited in this area.

Other observations: while the HPF had enough headroom to successfully drive the poweramp, turning up the tube preamp much past 9:30 induced clipping. On the other hand, whereas I had to turn the pre up to about 2:00 before, I was able to get a similar level at about 9:00 with the HPF, again with more clarity and dynamic range if a somewhat brittle feel.

I think that I was pushing the HPF close to its limits, here, but was surprised it could do this.

Any observations or insights are welcome. I'm still learning, and figure what I learn here can be applied elsewhere.

Thanks,
Mark

Question on a possible different use of the fdeck:

I picked up a little tube pre -- a simple, single 12AX7 affair with one, unbalanced output. Getting all kinds of 60-cycle hum with it. This might or might not have anything to do with an impedance mismatch, but the maker of the pre says that it needs to feed into an amp with a higher input impedance than its output impedance. Output impedance of the pre is about 40 KOhms. Input specs on my two poweramps are 20 KOhms for the one (an old Lab Gruppen fp2200) and 7.5 KOhms on the other (a Crest ProLite 2.0).

So, I'm wondering if feeding the pre into the fDeck, then the fDeck into the poweramp might work to rectify that. I have an HPF3. It's unity gain. Signal levels would be way hotter than anything coming from my pickups -- the Crest needs .75 V or so, and the tube pre will deliver more than that. I don't want to harm my HPF!

The other possibility might be trying one of those little passive transformers folks use inline with a mic to feed it into an instrument input. I think we have one of those lying around from my cassette tape 4-track days!

ADDENDUM:
Just checked some specs on the HPF-Pre homepage. Input range for Series 1 and 2 is listed as "1 V p-p with "musically forgiving" overload response". For Series 3 (which I have), "6 V p-p with hard clipping response". Maybe it would be okay with the tube pre, then? As I said, I know that it will put out way more than the .75 V the Pro Lite needs (and the Lab can take more than that), but 6V seems like it would be a safe margin. Don't know if the HPF's 10 MegOhm input impedance is what I need or not, and don't know anything about the output impedance.

Any input is appreciated.

Thanks,
Mark

 

[Turock]

Received my Series 3 today. Evey month or so, I play with a different band on a big hollow stage, with backline provided. It's a Peavey Mark somethng or other, with a 1x18 and 2x10 cabinet (one cabinet). It sounds terrible. It's boomy and some of the higher notes just disappear. I've already played with the amp's controls, tried two different compressors, Sansamp Para Driver, Sansamp BDDI and Carl Martin Para EQ. It still sounds like sh!t; lows boomy, highs disappear. I'm hoping the HPF will help... with the boom anyway.

I'm happy to report that the Series 3 HPF make quite an improvement in the sound of that POS backline. Sounded much better;clearer.

 

[ArtechnikA]

...it did not eliminate the 60 Hz hum.

I'd guess the unbalanced output from the pre is not helping you there - that's the kind of common-mode interference balanced lines are designed to overcome.
Did you try tweaking the filter? Turned "down" the two filters stack with a 35Hz knee, but you should be able to turn it up to where it starts cutting into that 60Hz.
Of course 60 Hz is the first overtone of a B string and well over the frequencies of both E and A, so if it's aggressive enough to knock back 60Hz it's probably also squashing all your low range...

 

[delta7fred]

Any observations or insights are welcome. I'm still learning, and figure what I learn here can be applied elsewhere.Mark

I cannot see how mismatched input or output impedance would cause hum. My guess is that there is a fault in your preamp and it just hums.

 

[ArtechnikA]

I cannot see how mismatched input or output impedance would cause hum. My guess is that there is a fault in your preamp and it just hums.

The kneejerk reaction to hum is bad power; I'd want to know what the pre was being powered by, and look for improvements there.

 

[MarkA]

Thanks, guys,

Power is a simple surge protector on my home line. Tried half a dozen other amps and pres on that line, though, using both balanced and unbalanced outputs, with zero issue. Also tried running the preamp on a separate outlet. Agree that the impedance mismatches were unrelated to hum, and that the cause is somewhere with the pre. It's since gone in for attention from its maker.

I'll have to look up "common mode rejection".

Are " all tube" pieces, which, I am informed, generally have an output impedance of around 40 KOhm unless there's some provision/additional circuitry to bring it down, susceptible to this kind of hum?

Are my power amps unusual in having input impedances of 7.5 and 20 KOhm? If the input impedance is supposed to be higher than the output impedance of the source (one person told me 10x higher, another 2x), how is such a piece to be used by your typical bass player with a power amp? Is it assumed that someone will use a DI or something else in between the pre and the amp, or am I missing something? Are there other factors I should consider?

Danke,
Mark

 

[delta7fred]

The kneejerk reaction to hum is bad power; I'd want to know what the pre was being powered by, and look for improvements there.

+1, or bad grounding scheme, bad layout, or faulty tube (especially if it has AC powered heaters).

We don't know anything about it other than it is single ended and has a 12AX7 so all this is speculation.

 

[ArtechnikA]

I've read a bunch of threads on power amp usage and I don't recall having seen input impedance having been mentioned even once.
I just don't think it's an issue once you get past the initial input stage. (But I am no expert...)
You'd like the amp input impedance to be "high" so it doesn't drag down the preamp's output. But preamp output is presumptively in the 0.75-1.4V range and not really much susceptible to loading effects, since it's buffered and all...
I've got a couple of pedals capable of being used as preamps (i.e. driving power amps directly, or the power amp section of an integrated head, via the Fx Return).
Your typical bass player plugs an instrument into a preamp, the preamp into a power amp, and ta da.

 

[Passinwind]

I've read a bunch of threads on power amp usage and I don't recall having seen input impedance having been mentioned even once.
I just don't think it's an issue once you get past the initial input stage. (But I am no expert...)
You'd like the amp input impedance to be "high" so it doesn't drag down the preamp's output. But preamp output is presumptively in the 0.75-1.4V range and not really much susceptible to loading effects, since it's buffered and all...

Not in the case of a tube preamp with the output coming off the plate of a common cathode circuit, al la F2B and quite possibly the one MarkA is discussing. Alembic does specifically mention amp input impedance as a potential issue in regard to the F2B.

Hum is a whole other thing and if you don't have total confidence in the builder's grounding and filtering scheme it may be really hard to speculate online about cures.

 

[ArtechnikA]

Not in the case of a tube preamp with the output coming off the plate of a common cathode circuit, al la F2B and quite possibly the one MarkA is discussing. Alembic does specifically mention amp input impedance as a potential issue in regard to the F2B.

Learned something today. Thank you.

 

[Passinwind]

Learned something today. Thank you.

De nada. Unfortunately, in the course of my DIY preamp building addiction I have learned about a whole slew of arguably questionable design choices in the commercial sector, let alone in the DIY world. But as usual, it's often mostly about tradeoffs. The F2B sounds really great in many situations, for instance.

 

[MarkA]

Not in the case of a tube preamp with the output coming off the plate of a common cathode circuit, al la F2B and quite possibly the one MarkA is discussing. Alembic does specifically mention amp input impedance as a potential issue in regard to the F2B.

Hum is a whole other thing and if you don't have total confidence in the builder's grounding and filtering scheme it may be really hard to speculate online about cures.

Well, running into the preamp straight vs. through the fDeck there was a substantial difference, with the latter being more dynamic and open and requiring a much lower setting on the pre to achieve the same volume. "Common cathode" is something else I'll have to look up, but conversation with the builder leads me to thing this might be the case. He mentioned some designs using a cathode follower. This isn't one. I, too, don't recall impedance being mentioned in matching up pres to poweramps, just sensitivity. If it's as uncommon a consideration as it seems to me, some mention should be made in the lit of the pre, but maybe this is something I should have been aware of?

As for the hum, we will see what the builder says. While dynamics, etc, seem related to the impedance issue, it seems apparent that the hum is not.

It's a bit complicated since I picked up the pre here from its original owner. Working with him and the builder to get things resolved. We'll see what happens. I don't want to derail this thread (though the HPFPre did come into play). I do appreciate the help and input.

Learned something today. Thank you.

As did I (I think). Thanks.

 

[BurningSkies]

Well, running into the preamp straight vs. through the fDeck there was a substantial difference, with the latter being more dynamic and open and requiring a much lower setting on the pre to achieve the same volume. "Common cathode" is something else I'll have to look up, but conversation with the builder leads me to thing this might be the case. He mentioned some designs using a cathode follower. This isn't one. I, too, don't recall impedance being mentioned in matching up pres to poweramps, just sensitivity. If it's as uncommon a consideration as it seems to me, some mention should be made in the lit of the pre, but maybe this is something I should have been aware of?

As for the hum, we will see what the builder says. While dynamics, etc, seem related to the impedance issue, it seems apparent that the hum is not.

It's a bit complicated since I picked up the pre here from its original owner. Working with him and the builder to get things resolved. We'll see what happens. I don't want to derail this thread (though the HPFPre did come into play). I do appreciate the help and input.

As did I (I think). Thanks.

I ran the HPF3 between my preamp and power amp for a while and found that even though the HPF3 is a 'unity gain' device there was a very noticeable 'bonus' in the level into the power amp. With the HPF in the middle I could turn the line out down by about 1/3rd of the sweep of the knob.

 

[Passinwind]

Well, running into the preamp straight vs. through the fDeck there was a substantial difference, with the latter being more dynamic and open and requiring a much lower setting on the pre to achieve the same volume. "Common cathode" is something else I'll have to look up, but conversation with the builder leads me to think this might be the case. He mentioned some designs using a cathode follower. This isn't one. I, too, don't recall impedance being mentioned in matching up pres to poweramps, just sensitivity. If it's as uncommon a consideration as it seems to me, some mention should be made in the lit of the pre, but maybe this is something I should have been aware of?

20K is not a particularly difficult load, so this may partially or all come down to something else. But since I can manage it fairly painlessly, here's a quick and dirty model of loading effect going from 5K-10Meg ohms amp input impedance, driven off a 12AX7 common cathode gain stage with 100K on the anode and a .1uF coupling cap, a la F2B:

The steps I modeled go: 5K, 10K, 20K, 50K, 100K, 500K, 1Meg, 10Meg. Gain was set arbitrarily to give ~ a +20dB reference level into a 10K load.

 

[StuartV]

Hi everyone, I have looked into the Fdeck HPF's before, but up until now I didn't think I had an issue that would cause me to need it. But now I think I do and I have some questions about it. I just bought a Ampeg SVT and a SVT-410HLF cabinet. On top of that it is one of the older SLM models. I think the cabinet sounds pretty good, it's just that it gets quite boomy very quick.

The 410HLF is just a boomy cabinet. They sound great when you play them by themselves. But, in a band mix, I think that's where you find out that they are just too boomy. This is not a problem to fix with an HPF. This is a problem to fix by getting a different cabinet. Well, that's my opinion, anyway. I had the same problem with my tube amps and an Aguilar GS410 (like, the SVT-410HLF, it's a 4x10 ported cabinet). I fixed it by changing to sealed cabinets with 10s (a Bergantino NV610 and an Ampeg 810, depending on the gig). The SVT-410HE (which is sealed) would be an option for you except that it's 8 ohm and the SVT amp only does 2 or 4 ohms.

I have an HPF 3 and I like it, but I don't think it's the right tool to fix the problem you're describing.

 

[SunnBass]

Who here has an fdeck mounted to the underside of a pedaltrain? I can't figure out a good way to do it.

 

[sunbeast]

Who here has an fdeck mounted to the underside of a pedaltrain? I can't figure out a good way to do it.

I use mine under a PT-Jr just using Velcro as I would on top and with the jacks or knobs facing out towards the side for easy access depending on the gig (in my main band I never adjust the settings, and so just have to plug the cord from my amp in). Another option for a more solid connection is to use Velcro straps.

 

[SunnBass]

I use mine under a PT-Jr just using Velcro as I would on top and with the jacks or knobs facing out towards the side for easy access depending on the gig (in my main band I never adjust the settings, and so just have to plug the cord from my amp in). Another option for a more solid connection is to use Velcro straps.

I just couldn't get a secure enough hold trying it that way...
Maybe you did it differently, can you post a photo?

 

[Woodsbass]

I ran the HPF3 between my preamp and power amp for a while and found that even though the HPF3 is a 'unity gain' device there was a very noticeable 'bonus' in the level into the power amp. With the HPF in the middle I could turn the line out down by about 1/3rd of the sweep of the knob.

I wonder if this is because the power isn't being wasted by the power amp to produce the deep lows so it doesn't have to be turned up as much? It could be unity gain, but just improving the efficiency of the amp I think!