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Chip Amps Amplifiers based on integrated circuits

Trying to replace my Carver Power amp
Trying to replace my Carver Power amp
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Old 21st November 2017, 09:11 AM   #1
Dedy is offline Dedy
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Default Trying to replace my Carver Power amp

Hey guys, My Carver TFM-35 power amp is dead and I was thinking of building a chipamp instead.

I've been reading a lot about the LM3886 and TFM7294 (I think..) but I am still puzzled about some issues since there are many designs/kits around the chips, some with opamps some with coupling caps and some without.

My current main system is as follows:

Carver C-19 Vacuum Tube Reference Preamplifier
Carver TFM-35 Power Amplifier (DEAD)
Quart 350S Two Way Spk 4Ohms

I know that the rated Wattage of the TFM-35 is way higher than the Chipamps but I never played it that loud anyway.

Since I'm not too familiar in Audio electronics I have a few questions:

1. Since I use my current preamp, is it better to go with a basic Chipamp design or one with Opamps?

2. For the LM3886 Gaincard design is specified as latest/best, is that so? and is it true that it does not use coupling capcitors? and... if so is it better than with coupling caps?

3. I would like to use my dead Carver amp's PSU and heatsinks etc. Its AC voltages are 40/73/130 VAC so which design best matches those?

4. I'm probably miising other important issues so please feel free to inlight me

Thanks for your help!!!
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Old 21st November 2017, 01:04 PM   #2
00940 is offline 00940  Belgium
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The service manual for the tfm35 is available here: Carver TFM-35 - Manual - Magnetic Field Power Amplifier - HiFi Engine

Honestly, the transformer in there is completely unsuitable for a chip amp (it's made for a high power class g amp). You'll need a new one.
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Old 21st November 2017, 01:18 PM   #3
bullittstang is offline bullittstang  United States
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Quote:
Originally Posted by Dedy View Post
1. Since I use my current preamp, is it better to go with a basic Chipamp design or one with Opamps?

2. For the LM3886 Gaincard design is specified as latest/best, is that so? and is it true that it does not use coupling capcitors? and... if so is it better than with coupling caps?

3. I would like to use my dead Carver amp's PSU and heatsinks etc. Its AC voltages are 40/73/130 VAC so which design best matches those?

4. I'm probably miising other important issues so please feel free to inlight me

Thanks for your help!!!
#1 - I use a pre-amp with a LM3886 amp and works fine, so I think you will be fine (with or w/o pre)
#2 - I just built a MyRef verC - boards are available on eBay. 2pcs DIY LM3886T LM3886TF diyAudio My Reference rev.C Power Amplifier PCB Board | eBay Was sceptical, but with a 2x22Vac toroid it is a very capable amplifier.
#3 - you could use the heatsink (mine is 7"Wx2-1/2"Hx2"fins with a 1/4" base and it doesn't even get warm) for a stereo pair of LM3886 boards.
The PSU - too much voltage for a chip amp - max is generally 25Vac for 8-ohm speakers.

I think you have the basics covered - just have to make a decision on the amplifier (which is always the hardest part).
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Old 21st November 2017, 02:36 PM   #4
Dedy is offline Dedy
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00940 Thanks for the info!

Bullitstang-
Thank you too for the useful data. Looked at the board, this one does not use Opas and speaker protection circuit, right? Does the Opa plays a major role?
(My understanding is that experimenting with different Opas can help improve the sound to one's liking)

As for the dead Carver,looks like only the case and heat-sinks are usable.
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Old 21st November 2017, 04:46 PM   #5
indianajo is offline indianajo  United States
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What continent are you on? Good transformer or switcher power supply vendors depend on that. In US Antekinc.com sells import power transformers with good reputation; in HGK connexelectronic.com sells switcher supplies which are more modern but require RF noise control.
As far as coupler caps in, I can't hear them & my hearing goes to 14 khz. I have a ST120 with caps in and out, a CS800s without, at same 1.5 W volume, on the same speakers, I can't hear the difference on the good channel. (one channel has low slew rate, slighty detectable). OPA op amps is another quibble I haven't dived into. I use FM radio a lot at a signal source, various LP's new & used, and very few LP's are in good enough condition or recorded well enough to stress my ST33078 op amps. I attend live performances and am an experienced musician, so I know what real instruments sound like. it took me 50 years in the hobby to afford speakers that even come close to live sound. The difference between .001% destortion & .0005% in the amp, PAH. Speakers cover all that up. 1% distortion is quite audible.
Serious heat sinks and maybe a fan are useful things to have, plus an enclosure. Hang on to the carver. Circuit breaker plus switch are handy, too.
What kind of music do you listen to? I listen at an average 1.5 W, but I like classical music, so on the cannon shot in 1812 overture, I like a 200 W peak. LM3886 won't quite do that. Rock pop EDM acoustic guitar have less dynamic range, so 1.5 - 10 w may be entirely pleasant. Acoustic piano needs 70 W anyway over 1.5 W average.
Two honey badger kit might fit in the Carver hulk, see link to diyaudiostore up in the top bar. This thread has various amp board recommendations where PCB are available www.diyaudio.com/forums/solid-state/314330-class-ab-amp-recommendations.html
Else there are various chip amp kits, atikita is one. I have suspicions of slew rate on LM3886 into 4 ohms particularly. 4 ohms takes more current. I hear slight fuzziness in bells, cymbals, top octave piano, on my AX6 board built with slow driver transistors.
Note to newbies, an output cap like the single supply atikita kit, can prevent you from burning up speakers if one solder joint pops loose and the output whangs to a supply rail. With split supply IMHO you need a speaker protection board like the $5 one being discussed in its own thread on solid state.
Have fun shopping & building.
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Last edited by indianajo; 21st November 2017 at 05:16 PM.
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Old 21st November 2017, 11:32 PM   #6
PRR is offline PRR  United States
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Quote:
Originally Posted by bullittstang View Post
....The PSU - too much voltage for a chip amp - max is generally 25Vac for 8-ohm speakers......
Disagree, maybe.

I never understood Carver's "Magnetic Field" designs. Apparently some had a "regulator" ahead of the power transformer.

But looking at the manual cited by 00940, this *appears* to be a conventional supply, albeit with three +/- rails 25V 56V 98V for a 3-level class "G" amplifier.

The +/-25V rails (which power the most-of-the-time part of the 350 Watts, to maybe 50 Watts) have two 6800uFd caps, ample (and upgradable).

If he takes just the +/-25V taps to an amp, that's maybe 28 Watts in 8 Ohms or 50 Watts in 4 Ohms. That can be a lot of good music.

It is "stupid" to have a 700+ Watt transformer sitting around to give only 100 Watts. But it isn't doing much else, isn't heavy enough to anchor a rowboat, and is already IN the house (not shipped from Canada or China).

The question is: can he find the indicated points, tap them, and not get shocked or burn down the house. Minor point: can heatsinks cut for individual transistors accept the wide chip-amp package.
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File Type: gif Carver-for-chipamp.gif (72.2 KB, 139 views)
File Type: gif LM3886-25V.gif (9.8 KB, 138 views)
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Old 22nd November 2017, 04:01 AM   #7
dotneck335 is offline dotneck335  United States
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Man up and fix the Carver!!
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Old 22nd November 2017, 11:09 AM   #8
indianajo is offline indianajo  United States
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Carver is way too complicated to learn diy repair on. I've looked at the schematic, and didn't buy one. I bought a dead peavey instead, which is a lot simpler. 127 parts later, I should have done the high voltage buffer after op amp trick and gone straight to predrivers, drivers, and new output transistors. op-amp based, high-gain, high rail voltage, preamp See post # 7
I could build this circuit with wire on mcmelectronics dip project board. I didn't really need the Peavey VI limiter or the flying ground which allows channel bridging. Just a serious mosfet speaker disconnect on excessive current, instead of the triac crowbar that melted the traces off the board instead of blowing the breaker.
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Last edited by indianajo; 22nd November 2017 at 11:23 AM.
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Old 22nd November 2017, 01:10 PM   #9
00940 is offline 00940  Belgium
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Quote:
Originally Posted by PRR View Post
Disagree, maybe.
You're perfectly right, I didn't realize that the voltages on the transformers were in fact 65-36-20-0-20-36-65.

The easiest, looking at pictures of the beast, would probably to rip out all of the pcb (we don't know where the fault is), keep only the transformer, big 10.000u caps and the heatsinks, bolt a diode bridge to the bottom, a pair lm3886DR on the heatsinks and hardwire all that.

Maybe get a small kit from ebay for the VU meters but that isn't trivial if one wants more than just decoration.

edit: a small caveat though. The pics and schematics are for the tfm35x. I don't know if it is much different.
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Last edited by 00940; 22nd November 2017 at 01:12 PM.
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Old 22nd November 2017, 04:21 PM   #10
Dedy is offline Dedy
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Well, thank you all for your feedback!

For starter, Indianjo- I'm from Israel. Love to hear various music types such Jazz, Rock, Soul & Classic among others.

PRR, thanks for the observation. I was looking at a printed schematics I have (see below) and took note of the ACV on the transformer which are wrong as they refer to the voltage between the transformer outputs. And BTW although it's called "Magnetic field..." the TFM isn't one. (lucky for me)

Looking at the pictures from 00940 ,the TFM 35 is different than the 35X, I have attached a photo of mine. So I think voltages don't match either. I need to check.

As for ripping off the Carver... I guess I cant go there mentally. I was thinking of maybe trying to repair it sometime as I have done before, but since its complicated design that includes feedback from one circuit to the other, failing to use the exact original transistors results in a kind of a domino collapse... which is what happen last time...resulting in a series of transistors and other components that gave up.
So that's how I got here. Thinking of building a Chipamp till I have the energy (and parts) for the Carver. After all I really love that beast.

Who knows, maybe I'll end up liking the Chipamp better?!!!

Ohh and about that...I spent so much time reading of different types of circuit designs etc till I got myself totally confused! Should I go for simple classic design or check things like My_Ref Fremen Edition ?
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File Type: jpg 2017-11-22 18.37.23.jpg (501.6 KB, 108 views)
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