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Old 9th June 2017, 09:05 PM   #1
ericj is offline ericj  United States
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Default Kicking a dead pig - upgrading low-end practice amps

A thrift store score cast me into learning how to play bass a month or so back.

I paid $60 for a Samick Corsair bass with hardly a mark on it and a Crate TB10 practice bass combo amp. Both circa 2004, both hardly used.

And then something dangerous happened. On a visit to a different thrift store i picked up a 50's or 60's Hammond Reverberation Speaker, which is just a Jensen P12P 12" alnico speaker in a blonde wood enclosure that looks like a small end table.

So. I cut the original connector and a foot or so of damaged wire off the end of the long hookup lead coming out of the Hammond box, took the back off the TB10, disconnected the internal 8" speaker, and jerry-rigged the Hammond to it.

Imagine my surprise when it not only sounded much better and fuller but was substantially louder.

This practice amp has a couple tl072, a ca3080 for the compressor, and a tda2030 with a small heatsink, powered by a 16-0-16 35va or so transformer. There are 7815 and 7915 to220 regulators with no heatsinks that i assume regulate power to the preamp, tone controls, and compressor.

So, Crate's advertised spec of 30w through the 8" 8-ohm original speaker is more than suspicious. It's pretty much a lie.

But it's not a BAD practice amp. It's not noisy, the compressor works real well, just the speaker sucks and the power stage is kinda limited.

So it turns out that Parts Express has an 8" woofer that has a claimed FR from 40hz to 7khz, 95db at 2.83v/1m. 90w RMS power handling. Best part? recommended sealed cabinet volume of 0.29 ft^3 for an F3 of 109hz.

So it should perform well in a cabinet right in the neighborhood of size that i already have. Though Crate built it vented, so I'll have to seal it up. At least, I don't think you can really call it ported when there are just two 1.5" holes in the baffle.

I am fully aware that a *guitar speaker has properties, some real and some imagined, that make it a useful tool for the production of "tone".

I'm pretty sure that bass speakers are more similar to high-fidelity speaker designs. Or at least, that the needs of a bass player can be well met by a speaker with a reasonably flat response from 100-3000hz or so. Particularly given the presence of tone controls. It's apparent that many bass combo amps have essentially full range response - if voiced a bit dark - and often with the ability to drop the treble 6db or entirely.

This is no longer my only bass amp. It's not even the second best. So I'm prepared to eff with it.

I went ahead and bought that woofer from P-E. I'm about ready to pull the trigger on some stuff from Apex Jr including their cheap tweeters, which should match this woofer pretty well. I have plenty of crossover network parts.

I have a couple 18-0-18 transformers that have a VA size near 80-100.

I've ordered what i hope are some genuine TDA2050 parts.

It seems like if i upgrade the transformer, the main rail capacitors (and any others requiring further safety margin), install rectifiers that can handle more current, adjust the R-C network to match the 2050 spec, and tweak the feedback resistors for higher gain, I might actually meet or exceed that 30w spec.

To tell the truth, since this woofer is rated for a peak of 180w, I think it would be kinda hilarious to try and make it as loud as i can. 100w practice amp that fits under an airline seat, anybody?

Anybody else been down this rabbit hole? Obviously not with the same amp.

I have a Gorilla TC-35 guitar amp that is actually pretty alright with all of the caps upgraded to modern low-ESR parts. Might give it the same chip and power supply upgrade.

Last edited by ericj; 9th June 2017 at 09:14 PM.
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Old 9th June 2017, 09:29 PM   #2
mozz is offline mozz  United States
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I wouldn't play bass through a old jensen p12p. That being said, i wouldn't even try to upgrade the Crate. Seems like throwing money away.
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Old 9th June 2017, 09:39 PM   #3
ericj is offline ericj  United States
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Quote:
Originally Posted by mozz View Post
I wouldn't play bass through a old jensen p12p. That being said, i wouldn't even try to upgrade the Crate. Seems like throwing money away.
I don't plan to play bass through the P12P. Right now the idea for that speaker is a small single-ended tube guitar amp, built into the Hammond cabinet.

As for trying, I have disposable income and it's up to me how to dispose of it, isn't it?

The preamp, tone stack, and compressor built into it work very well.

Last edited by ericj; 9th June 2017 at 09:41 PM.
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Old 9th June 2017, 10:48 PM   #4
ericj is offline ericj  United States
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More to the point, if it works and i find it amusing and/or useful, how would that be throwing money away?

Aren't we here because we LIKE tinkering with audio electronics?

People throw money away on old cars all the time. They'd be better off buying a cheap modern mid-range car that is better in every way.
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Old 10th June 2017, 12:56 AM   #5
JMFahey is offline JMFahey  Argentina
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I was typing a presumably useful answer, but given your tantrum .... sorry, your "attitude" .... , ok, go ahead, itīs your money, burn it uselessly any way you want.
Itīs your speaker too, so also burn it in the same pire, you can save 1 match.

By the way, Crate didnīt lie
Quote:
So, Crate's advertised spec of 30w through the 8" 8-ohm original speaker is more than suspicious. It's pretty much a lie.
The spec sheet claims "output power rating: 15W"
Click the image to open in full size.
Sorry you canīt read a spec sheet.
Quote:
100w practice amp that fits under an airline seat, anybody?
In your dreams
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Old 10th June 2017, 01:32 AM   #6
Enzo is offline Enzo  United States
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Agree^^^ Crate didn't lie, the 30 watts on the thing refers to how much power it USES from the wall outlet to make its rated 15 watts. Also, the speaker says 30 watts by it meaning the speaker can HANDLE 30 watts, not that the amp produces 30 watts.

Hooking the amp to a larger speaker in a larger enclosure is louder I do not doubt. Your old Hammond speaker is more efficient, probably in more ways than one, so it is louder and deeper. There is a limit to how deep and full range an tiny 8" speaker can go.

So many folks have discovered that their tiny practice amps sound so much bigger when played through a 4x12 cab.

By the way, if you were to upbuild your power supply and install a TDA2050 to double the 15 watts to 30 watts, the result would only be 3 decibels louder. Not much.
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Old 10th June 2017, 11:37 AM   #7
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I picked up a Crate G20XL guitar amp at a hamfest for $5. It looks like that it, and the other $5 amps I got with it were the victims of a previous upgrader. I fixed the Fender Gdec, played with it for a month or so then gave it to a friend.

I will "upgrade" the Crate. I however go at upgrading from a different angle. The crate has a nice cabinet, a decent reverb tank, and a speaker of unknown usefulness. $5 for a cabinet and tank is OK in my book, everything else gets tossed. I think that open backed box is asking me for about 20 watts of screamin tube power and a "vintage" toned speaker.......

I wander the hamfests and flea markets for dead or otherwise cheap guitars, amps and music keyboards that have full size keys, usually in the under $10 range. I look at them all as parts donors.

Note that the speakers efficiency plays a large part in the volume an amp can produce. A 5 watt amp through a 100 dB speaker is about as loud as a 50 watt amp through a 90 dB speaker.
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Old 10th June 2017, 07:07 PM   #8
ericj is offline ericj  United States
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Quote:
Originally Posted by JMFahey View Post
I was typing a presumably useful answer, but given your tantrum .... sorry, your "attitude" .... , ok, go ahead, itīs your money, burn it uselessly any way you want.
Itīs your speaker too, so also burn it in the same pire, you can save 1 match.
Tantrum? Sorry. Irks me when someone comes to my thread just to tell me they don't approve of it.

Quote:
By the way, Crate didnīt lie
The spec sheet claims "output power rating: 15W"
Sorry you canīt read a spec sheet.
OK, read it wrong i guess. 15w is even a little hard to believe.

Quote:
In your dreams
Expensive but plausible. The hard part is the power supply. There aren't a whole lot of bipolar switching power supplies with the right specs to power a single irs2092 board. There's one that came across parts express several months ago built more for at least two of them.

Quote:
Originally Posted by Enzo View Post
There is a limit to how deep and full range an tiny 8" speaker can go.
There's a limit, but it's further out than most people believe. You don't need a 12" or bigger transducer to reproduce the lowest frequencies a human ear can reliably hear. It's easier, but not required.

Quote:
By the way, if you were to upbuild your power supply and install a TDA2050 to double the 15 watts to 30 watts, the result would only be 3 decibels louder. Not much.
Also going from 8-ohm to 4-ohm speaker. ST claims "the TDA2050 is able to provide up to 35 W true RMS power into a 4 ohm load at THD = 0%, VS = ą18 V, f = 1 kHz and up to 32 W into an 8 ohm load at THD = 10%,
VS = ą22 V, f = 1 kHz. Moreover, the TDA2050 delivers typically 50 W
music power into a 4 ohm load over 1 sec at VS = 22.5 V, f = 1 kHz"

If i achieve 6db, that is twice as loud, no?

Quote:
Originally Posted by Tubelab_com View Post
Note that the speakers efficiency plays a large part in the volume an amp can produce. A 5 watt amp through a 100 dB speaker is about as loud as a 50 watt amp through a 90 dB speaker.
Of course. My experience so far with this junk suggests that the speaker i picked up from P-E is more efficient than the one i pulled out. Looking at the specs for replacement guitar speakers from jensen, celestion, and eminence in the 8" size, they seem to have similar db/v ratings to the anonymous $18 P-E woofer.
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Old 10th June 2017, 10:37 PM   #9
Enzo is offline Enzo  United States
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No, 6db is not twice as loud. 10db is twice as loud. And 10db is ten times the power. SO it takes 100 watts to be twice as loud as 10 watts, if all else is the same.

efficiency is the 1 watt at 1 meter reading in the spec sheet.

Assuming your new 4 ohm speaker was otherwise identical to the 8 ohm, it would try to draw twice the current from the amp at a given output voltage, and that doubles output power. The difference between 30 and 35 watts is negligible.

Quote:
There's a limit, but it's further out than most people believe. You don't need a 12" or bigger transducer to reproduce the lowest frequencies a human ear can reliably hear. It's easier, but not required.
Well, this is conflating theory and practice. Can you use a small speaker to make really low notes? Yes, they call them subwoofers. You may have one for your TV sound system. But to do that we use a big flappy surround for lots of excursion and put it in an enclosure especially designed for that. But neither that type of speaker nor that type of enclosure are found in small practice amps. Ever try playing guitar through a subwoofer unit? Your basic 8" guitar speaker just won't hack much bottom end. Not unless you want to hop the voice coil out of the gap.
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Old 11th June 2017, 07:33 AM   #10
ericj is offline ericj  United States
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Quote:
Originally Posted by Enzo View Post
No, 6db is not twice as loud. 10db is twice as loud. And 10db is ten times the power. SO it takes 100 watts to be twice as loud as 10 watts, if all else is the same.

efficiency is the 1 watt at 1 meter reading in the spec sheet.

Assuming your new 4 ohm speaker was otherwise identical to the 8 ohm, it would try to draw twice the current from the amp at a given output voltage, and that doubles output power. The difference between 30 and 35 watts is negligible.
6db is double the sound pressure. Do we need to have an argument about how much fletcher-munson matters?

30 and 35? I thought we agreed that the original spec is really 15w.

My best bass amp at the moment is a Behringer BXL450, rated at 45w with a 10" Bugera speaker driven by (drumroll) a tda2050 with a generous heatsink, and it's way the hell louder than the crate. I haven't found a source for specs for the Bugera speaker yet, since that's basically a berhinger house brand.

Quote:
Well, this is conflating theory and practice. Can you use a small speaker to make really low notes? Yes, they call them subwoofers. You may have one for your TV sound system. But to do that we use a big flappy surround for lots of excursion and put it in an enclosure especially designed for that. But neither that type of speaker nor that type of enclosure are found in small practice amps. Ever try playing guitar through a subwoofer unit? Your basic 8" guitar speaker just won't hack much bottom end. Not unless you want to hop the voice coil out of the gap.
I am not using a basic 8" guitar speaker.

I'm using an 8" midwoof that appears to have been designed for a small 2-way bookshelf design since it extends from 40hz to 7000hz. F3 (self-resonance) of 109hz in a sealed enclosure of 0.29 cubic feet.

Or F3 of 40hz with a 2" diameter 5" long port in .79 cubic feet.

my casual observation to date is that bass practice combos generally have a closed back, so i thought that was generally understood. it will be trivial to seal the TB10 enclosure like a drum. it may actually have more than .29 cubic feet of internal volume.

I'm considering pairing it with a tweeter of similar efficiency that is best crossed over at 3000hz. And adding a switch that can disable the tweeter. I may or may not put a 3.5khz 3db low-pass on the woofer, maybe install it along with a bypass switch for flexibility.

I have most of the stuff i need to measure speaker impedance curves and frequency response. I'm not above nefarious methods to shape the response curve if necessary. I realize there are limits to how you can shape the response of a speaker, but i've seen measured response curves of a whole lot of "cabinet simulator" circuits and dsp systems, and it sure looks like the classic bass amp has not much response below 80hz, a small hump at about 100hz, a small dip in the neighborhood of 200-400hz (middle of the piano keyboard region), and generally falls off a cliff somewhere between 3khz and 6khz.

Last edited by ericj; 11th June 2017 at 07:49 AM.
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