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Class D guitar amp
Class D guitar amp
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Old 9th February 2019, 10:06 AM   #1
pettol is offline pettol
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Default Class D guitar amp

Iíd like to build a class D guitar amp. I know this has been discussed before, but I havenít found the answers to my questions. Some background:
- The aim is approximately 100 watts output power (@ 8 ohm).
- I know overdriven solid states sound awful, so the guitar amp tone and overdrive will come from the preamp.
- It will be combined with whatever cabinet available, which in practice means it needs to handle 4-16 ohm speakers.
- I think good candidates could be TDA7498 and TDA8950 (or 54), for which pre-assembled boards are readily available.
So,
1. Are there better amps (than the aforementioned chips) for this?
2. Apart from different output powers and the potential need for output filter adjustment, is there a problem with hooking up the amps to speakers with different impedances (4-16 ohm)?
3. Iím planning to include a switch to select between to sets of inductors and capacitors for the output filter. Is that a bad idea? Is it necessary?
4. If the chip is designed for stereo, is it OK it leave one channel unconnected (input as well as output)?

Thanks in advance!
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Old 9th February 2019, 10:49 AM   #2
Khron is offline Khron  Finland
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My first question is, especially for guitar (ie. not bass), do you really NEED 100W?

Let's keep in mind that guitar speakers, even in singles, have efficiencies in the mid-to-upper half of the 90dB @ 1W area. I guess the question is, how many people do you want to annoy, and how much?

For what it's worth, my band's guitarist, with a little TPA3118-based amp that i put together (powered with a 19v power brick, resulting in ~40w "clean"), got an Ibanez 4x12 4ohm cab up to about 117dB at around 2m in front of the cab, at full blast. That was pretty effin' LOUD despite wearing moulded earplugs. The master volume around half-way was enough to keep up with the drums right next to it.

Stereo chips can usually be configured as PBTL (in most cases, anyway).
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Old 9th February 2019, 11:06 AM   #3
FauxFrench is offline FauxFrench  France
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Khron and voltwide are very competent with guitar use.

TDA7498E or TDA8954 may be good candidates. TPA3116 can be sufficient.

In principle an output filter is optimized for the speaker impedance. But, if you use a filter for 8 Ohm and actually use a 4 Ohm speaker the filter will just be damped a little more. Make sure the chokes can handle the 4 Ohm current. With 16 Ohm the filter will be a little less damped. You will hardly hear the difference.

Two different sets of filters will require very reliable connections because loosing the load on an active class D amplifier may cause damage.

You may leave one channel unused but only with a dummy load at the output of the filter or with that filter disconnected from the chip output.
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Old 9th February 2019, 11:31 AM   #4
voltwide is offline voltwide  Ireland
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1. I do not expect significant sound differences between class-d-chips. Select them according to your power requirements and the losses they produce. Chose operating voltage depending on max power and impedance. For instance 36V for 80W/8Ohms, 24V for 40W/8Ohms, 12V for 10W/8Ohms.
Over the years I decided there is no need for 100W guitar amps for me and now I use most of the time 24V/8Ohm with TPA3118.

For smaller venues 16V (4xLiIon) 4 Ohm with a 6.5" Jensen MOD15.
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Old 9th February 2019, 11:48 AM   #5
jazbo8 is offline jazbo8
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Class D guitar amp
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Old 9th February 2019, 12:24 PM   #6
pettol is offline pettol
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Thank you so much for your input!

@Khron. Do I need 100 W? Very good question! Honestly, I don't know. I'm actually building this for a friend who's gonna use it for playing live at small venues (which is rather undefined . We decided to go for 100w which would give some power and headroom even at 16 ohm. But maybe the Tpa3116/18 is enough. I'll think about it.

Thanks again and sorry for posting on the wrong forum.
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Old 9th February 2019, 12:39 PM   #7
Khron is offline Khron  Finland
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"Back in the day", people like Pete Townshend (of The Who) began to need those amounts of power because there wasn't yet much concept of what's PA today. And what with playing in bigger venues, they needed the SPL to cover the larger crowds.

Nowadays though, even in small(er) clubs, you might have your cab mic'd up and put through the PA, so "worst case" you need a bit of volume (and thus, power) just for some amount of monitoring.

My aforementioned bandmate got himself a 120W Laney Ironheart head a couple years back, but recently bought himself the 15W version, and even that's more than plenty loud for rehearsals, with that same 4x12. Just sayin'
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Old 9th February 2019, 03:06 PM   #8
Printer2 is offline Printer2  Canada
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Every time you double the speaker area you get a 3 dB increase in output, so a 4x12 would have a 6 dB advantage over a single 12" (using the same speaker).
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Old 9th February 2019, 04:05 PM   #9
voltwide is offline voltwide  Ireland
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Quote:
Originally Posted by pettol View Post
Iíd like to build a class D guitar amp. I know this has been discussed before, but I havenít found the answers to my questions. Some background:
4. If the chip is designed for stereo, is it OK it leave one channel unconnected (input as well as output)?
Thanks in advance!
Should be no problem. You may short circuit the input of the idle channel.

Better you configure the stereo amp in parallel BTL-mode, that reduces losses and spreads heat dissipation on both channels.
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Old 9th February 2019, 04:20 PM   #10
Printer2 is offline Printer2  Canada
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Some boards are already configured as BTL.
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