THD Hot Plate - Line Out - Can it be improved?

[led-hendrix]

There seems to be a lot of people on various forums who dislike the line out of the Hot Plate. It's usually described as sounding too thin.
There is the possibility that some people didn't realize the Deep and Bright switches should be turned off when using the line out, but I'd still tend to agree that it could sound better.

Here's a diagram of the 16ohm Hot Plate line out:



What I'd like to find out is whether the following line out circuit would be an improvement in terms of sound quality since it doesn't use the pot as the voltage divider.



Is it more likely to just be the dummy load that's having a detrimental effect on the tone of the line out?

[J M Fahey]

The real problem is that all the attenuators you show are "flat", so lacking the speaker "filtering" we hear.
I bet the best solution would be to add a good EQ between Hot Plate line out amd mixer line in, and tweak to taste.
Or use a speaker emulator, but I guess that a 31 band EQ (or at least 15), although harder to set up initially, will be able to get real close to what you have in mind.

[led-hendrix]

Thanks J but when I wrote 'sounding too thin' I meant into another amp or after a cab impulse response.
Any ideas as to whether the line out circuits would even sound different from each other?

[J M Fahey]

OK:
1) the idea of adding EQ is *still* valid (of course) whether you send it "into another amp or after a cab impulse response.
Your problem is that you don't like what you hear ... EQ to the rescue.
2) out of the schematics you posted, I'm quite sure the upper one is wrong. *Weird* values.
And if that's *really* into a Hot Plate .... well, you read my comment about it.
The 2nd one looks better, but still if connected to a 100W amp driving a 16 ohm cabinet (40V RMS) it can out out 7V RMS.
"Hotter than H*ll".
It can be padded, of course, but the 5K *linear* pot does not have much smooth range.
And both schematics are flat , so they sound the same.

[Steve Conner]

A cabinet impulse response is the same thing as an EQ.

I've had good results running my amp into a simple dummy load followed by a digital parametric EQ and a reverb. I fiddled with the EQ until it sounded good. It ended up with aggressive low-pass filtering and lots of peaks and notches.

The reverb is important to give the impression of playing in a room.

[led-hendrix]

Thanks for the responses.
EQ can definitely be used to beef up the low end, but time and time again I've found it's best to get things correct at source. Eq can't perfectly fix everything unfortunately.

I'm not all that knowledgeable in electronics but I thought the Hot Plate diagram shows the pot being used as the voltage divider. So technically it can put out less than 1 volt if you set it in the correct position. The other diagram has a resistive voltage divider and then the pot to further fine tune the output voltage.

Thanks for the comment about them being flat, that's what I wanted to know. I guess the people who disliked the line out of the Hot Plate didn't know about the 'Deep' and 'Bright' switches which need to be turned off.

[J M Fahey]

Doyou have a full hot plate schematic?
So we can check the line out stuff, the hi/lo switches, etc.
Otherwise we are in the daRK.

[led-hendrix]

I'm afraid I don't have the time at the moment to map out the whole thing, and I don't think THD would appreciate it much.

If you are correct that the two schematics are going to sound close to identical, then it is obviously a combination of the dummy load and the two EQ switches being left on that people had an issue with. Since I have the two EQ switches off, the dummy load itself must produce a slightly lean low end or take some low end away from the line out. I think all attenuator line outs will sound slightly different as they all must interact with the dummy load in a small way.

[Steve Conner]

A Marshall 4x12 has a monstrous bass resonance at about 120Hz that gives it its big chunking sound. Emulating that in a passive circuit needs a big inductor, so it is quite likely that THD left it out. In this case I think a parametric EQ really is the answer.

[J M Fahey]

Agree and add: led-hendrix keeps referring to those "Deep" and "Bright" switches.
Since we don't have the schematic nor know what they do, what can we say?

[led-hendrix]

Correctly made impulses (especially those made with a valve power amp rather than solid state) do include the impedance peak in the very low frequencies and the bass build up in a 4x12 cab, so I don't necessarily think that would be a problem.

Just to make things clear, when turned on the two switches leech through some non-attenuated signal in the lows and highs. This unfortunately reduces these frequencies from the line out unless they're turned off, so as I stated in my previous post I have these turned off so that they're out of the equation.

The important information was that the line out circuits will not sound different from one another. It must be the dummy load at full attenuation that sounds a little lean in the low end and harsh in the high end, so there would be no point in modding the line out.

Thanks for the help.

[Steve Conner]

This all sounds very unscientific, have you personally compared the sound of the Hotplate with the sound of another dummy load, or are you going by Internet rumours?

Getting things right at the source? There are two sources, the amp and the cabinet impulse. If the impulse sounds harsh and thin (all too easy to achieve by close micing with a SM57) then it'll make everything you put through it sound harsh and thin.

If the impulse was made with a valve amp, and the Hotplate also models voice coil inductance, you could be getting a double dose of the presence boost caused by voice coil inductance interacting with the amp's output impedance. That could well sound harsh.

The two circuits in the original post should behave the same, except one will have a higher output impedance. That might interact with your computer audio interface or whatever device you are plugging it into.

[led-hendrix]

Maybe I should have mentioned that I used to use a Weber attenuator which sounded more robust in the low end and less muddled/harsh in the high end (closer to my real speaker recordings), hence "I'd still tend to agree that it could sound better". I was originally under the impression that the line outs could sound different from each other, but you have confirmed that they shouldn't sound different.

I've used many impusles and some accentuate the flaws in the Hot Plate and some compensate for them.
The dummy load along with the guitar and amp are sources of the recorded tone, the impulse is just a post filter which isn't a permanent part of the tone.
I use impulses made with a solid state amp (Red Wirez, Two Notes Torpedo, etc) so there is no doubling of the speaker's impedance curve to worry about.

Thanks for confirming that the line out circuits should behave the same. I use a Hi-Z/line level input that can cope with the different output impedances. I'll just try to get a hold of a weber attenuator again, or maybe something even better, and do some more testing.

[Steve Conner]

Was that the Weber MASS? The reason I ask is that it has a speaker motor assembly inside that models the bass resonance of a real cabinet. So when used with a tube amp, it would indeed have a more robust low end.

Also, many solid-state guitar amps have a high output impedance, so when making impulses with them, you would get a doubling of the impedance curve. You would have to use a PA or hi-fi amp to be sure.

My philosophy: "Flaws" are for audiophiles. In musical instrument work there is only tone you like and tone you don't.

[led-hendrix]

That's the one. I'm sure now that I just happen to prefer the fully attenuated tone of the MASS over the Hot Plate. The forum posts about the line out of the Hot Plate just mislead me, some were even suggesting to build a small line out box with the diagram I posted.

The two companies I mentioned definitely use Hi-Fi solid state power amps. Redwire even needed to release an impedance curve IR for those using digital amp sims.

Thanks again for your help.