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Old 12th June 2018, 02:14 PM   #91
THRobinson is offline THRobinson  Canada
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Ya... I moved here from Guelph, big change, such a small... country music lovin'... area. Bleck. I'm more into 70/80's rock. Never learned blues and don't really listen to it much but always liked how it sounded with the acoustic. May start learning some of that.

PSU's... I do have a few laptop power supplies, I think ranging from 18-24v, not sure the wattage/amps though off-hand. Had one set aside for a $6 eBay board, 19v but ended up not using the board for the project. Still plan to do a cap swap and probably use it for something else. Quality was meh... heatsink also barely on, holes were off a bit as well so sat crooked, volume control could have been mounted a bit better as well.

I did find the spec sheet for that chip actually, I guess 2 versions, a stereo one and a mono one under TDA7498MV

Features
• 100 W output power at THD = 10% with RL = 6 Ω and VCC = 36 V
• 80 W output power at THD = 10% with RL = 8 Ω and VCC = 34 V
• Wide-range single-supply operation (14 - 39 V)
• High efficiency (η = 90%)

Lower down it has a spot in a chart for 10%/1%... but no mention of power supply, ohms or anything. I guess 100w with 10% THD with 36v and 78w with 1% with 36V both with 6ohm?

Click the image to open in full size.

I've seen some amps listed where they've listed 10% THD at Xvolts, and 1% THD at Yvolts... so think I got my math mixed up reading too many eBay China mis-specs.

So I guess with 8ohm... 80w with 10% THD using 34v. From your chart before, I guess 10% 80w, around 60w 1%? Hmm... maybe go cheaper on the amp, and pay the $30 for the 32v PSU.

Though I will admit... I'm fine with cheap eBay China parts like the amp, power supply however is a bit more of a fire hazard. If I go 24v, may be better to go to DigiKey and get a 24v Mean Well brand one for $10 more.

LRS-100-24 Mean Well USA Inc. | Power Supplies - External/Internal (Off-Board) | DigiKey

Going from computer power supply horror stories, it has me debating.

Quote:
I used that one in a previous project. Beware the weird input (audio) connector - I believe it's a "JST-XH", which has 3 pins spaced 2.5 mm apart, and it doesn't come with the amp board, so you have to source it separately. Guess how I found that out.
Luckily I'm not too worried about desoldering stuff. Likely desolder it and solder the wires direct to the board. Desolder the pot as well, solder in a foot of wire and run to the front of the cab same as the LED. Though may upgrade the pot to something bigger, or, a slide pot. Kinda like the idea of a sliding volume control.

So, that board should work and a few with 0 bids for $13CAD. Also saves me the trouble of sorting out where/how to add a volume pot which is a bonus.

I guess now it's 24v or 32v PSU. Brand name or no-name.

So, when picking out this stuff... how do you know which chips are better than others? for example.. the TPA3116DA I linked, could it be a better choice than the TDA7498?

Last edited by THRobinson; 12th June 2018 at 02:17 PM.
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Old 12th June 2018, 04:27 PM   #92
voltwide is offline voltwide  Ireland
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I do not know what is "better".
All I can say that 36V is louder than 24V
And TPA3118 is limited to 24V
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Old 12th June 2018, 04:46 PM   #93
THRobinson is offline THRobinson  Canada
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so... here's an onion in the ointment...

NextGen did mention the best for the modelling is a PA speaker cab, woofer+tweeter/horn.

In a guitar forum I was also asking for advice on speakers to see what they'd recommend and was mentioned as well, better to have a woofer+tweeter.

That Eminence is a PA speaker so I assume will handle what it needs for mids/lows. Would it make sense to add in a dome tweeter and a crossover?

Which kinda messes up things a wee bit.... 2 speakers and such.
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Old 12th June 2018, 04:50 PM   #94
THRobinson is offline THRobinson  Canada
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Quote:
Originally Posted by voltwide View Post
I do not know what is "better".
All I can say that 36V is louder than 24V
And TPA3118 is limited to 24V
Simple and straight forward... I'm good with that.

Quality wise though I guess so long as within the parameters (ohm, volt, etc) they're all about the same.
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Old 12th June 2018, 06:46 PM   #95
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by THRobinson View Post
Quality wise though I guess so long as within the parameters (ohm, volt, etc) they're all about the same.
You know how discussion of religion and politics is banned on this forum because people tend to get very emotionally engaged on those topics?

Discussions of audio amplifier distortion aren't banned, but it seems to stir up equally crazy levels of emotion for some true believers.

The science is pretty simple: at least five decades ago, it had been thoroughly demonstrated in well-conducted double blind listening tests that few if any people could detect 1% THD in music, though some could detect it when added to a pure sine wave test tone. Drop that to 0.5% THD or less, and nobody could detect it on any kind of signal (music or test tones) with any statistical significance.

All the chips we've discussed on this thread dip down to around 0.1% THD over much of their output power range, and stay there until they get close to clipping, at which point the distortion curves switch from being nearly horizontal, to nearly vertical.

To get bigger power numbers, the manufacturers push these chips far into clipping (10% THD). But IMO this is marketing at its dumbest, ignoring an important spec (inaudibly low levels of distortion) for an unimportant one (how loud it can get if you clip the heck out of it.)

The important point being: as long as you keep them out of clipping, all these chips have distortion levels too low for the human ear to detect. Distortion-wise, they are all essentially perfect as far as our ears are concerned.

This applies to almost every home Hi-Fi amp sold in the last four decades, too. I used to be interested in Hi-Fi audio amp design, until I realized that any old $40 thrift-store receiver from the 1990's (in good working condition) is audibly perfect as far as distortion goes; keep it out of clipping, and you can't improve on its sound with any amount of money.

Since the science of audio / audio perception was 95% settled by 1970, most actual researchers and scientists had left the field by then. The resulting vacuum was filled mostly by audiophiles - which usually translates as "a person with deep emotional involvement with audio electronics and hardware, who rejects the scientific method completely, and may not have any real technical knowledge."

These people continue to believe that you can hear 0.0005% distortion, that red plastic insulation makes a wire sound different than black plastic insulation, that wooden discs made out of mpingo have magic properties ( The Magic of Mpingo ), and so on.

But if you don't belong to their crazy-making world, Voltwide's approach makes complete sense. All modern audio amps have perfect distortion levels (too low to hear.) So choose by price, power output, and reliability, if you have any information on that. Maybe signal-to-noise ratio, if you happen to find an amp noisy enough to not be perfect in that department as well.

Amps are essentially audibly perfect, but speakers - and their interaction with rooms - are very far from perfect. One of the things that is a head-scratcher for me is that some people obsess about 0.01% THD in their power amplifier - while ignoring the 10% THD from their woofer!

-Gnobuddy
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Old 12th June 2018, 07:06 PM   #96
chris661 is offline chris661  United Kingdom
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Quote:
Originally Posted by THRobinson View Post
so... here's an onion in the ointment...

NextGen did mention the best for the modelling is a PA speaker cab, woofer+tweeter/horn.

In a guitar forum I was also asking for advice on speakers to see what they'd recommend and was mentioned as well, better to have a woofer+tweeter.

That Eminence is a PA speaker so I assume will handle what it needs for mids/lows. Would it make sense to add in a dome tweeter and a crossover?

Which kinda messes up things a wee bit.... 2 speakers and such.
You don't need a tweeter. The reason I recommended the LT version is because it has a whizzer cone which extends the high-frequency response high enough for your purposes.

Chris
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Old 12th June 2018, 07:08 PM   #97
THRobinson is offline THRobinson  Canada
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That's right... I knew there was a reason for the LT over the other model... and I was certain it was in this forum that I read it but, couldn't for the life of me find it.
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Old 12th June 2018, 09:26 PM   #98
voltwide is offline voltwide  Ireland
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From my point of view a two-way guitar cab with passive x-over is quite a bad idea, because:
When the amp is driven into clipping by frequencies of let us say some 300Hz, this tweeter will reproduce all nasty harmonics up to 20kHz - and this is exactly what gnobuddy called the "cazoo" sound.
A real guitar speaker has a very sharp roll of above 3..5kHz - so this will never happen with a guitar speaker.
And this is an audible difference you CANNOT equalize with any DSP in the signal chain.
So, if a tweeter is on your wishlist, you should consider bi-amping as well.
Or a very good limiter that avoids clipping of the power stage.

Last edited by voltwide; 12th June 2018 at 09:36 PM.
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Old 12th June 2018, 11:46 PM   #99
dotneck335 is offline dotneck335  United States
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You might have some benefit of a Hi-Fi type of approach for an acoustic guitar; but NEVER for an electric guitar. I have never heard an electric guitar sound good through a Hi-Fi speaker, no matter what processing is put in the preamp.
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Old 13th June 2018, 02:06 PM   #100
THRobinson is offline THRobinson  Canada
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I think I am good to go... few tweeks in the Sketchup design, make sure everything fits nice and tidy... speaker size a tad different, but internal volume of the cab is within specs of what that speaker recommends. Then order some parts this weekend.

Down to the debate of 30w or 50w...

Plan to make the cab from 3/4" Plywood (birch probably). Inside, I'll use 1/2" strapping. Hoping for some 3/4" thick accenting wood on the front for the volume/light/power switch, like mahogany, but small cheap bits are hard to find, being exotic/endangered woods and such.

I think overall (not at my home PC) the design is 22"Wx18"Hx14"D... with 1/3 of the back being open.

Sadly, no 1 place has everything I need... trying to find a cheap amp board in N.America else given my usual experience with eBay China (or Amazon China nowadays) I may not have this built until August.

Last edited by THRobinson; 13th June 2018 at 02:13 PM.
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