Beginner's Gainclone, HiFi LM1875, The Amplifier Board

[lausar]

Originally Posted by danielwritesbac

I think that I've seen those. It should be fairly easy to replace the single turn trimmer (looks like a dial) with a multi-turn trimmer (looks like a little box with a little screw on it), same for both rails. Then you can precision adjust the rails to identical voltage.

Any mixed/direct coupled amplifiers (amplifies DC by the gain factor) could have a rather bad accident if one power supply powers up before the other in a split rail SMPS scheme (really gigantic offset voltage results). Actually, we should probably consider a reliable center tap transformer and ordinary (or CRC) linear power supply for the amplifier on this thread.


P.S.
My TDA729X projects will easily tolerate SMPS.

After you sent me some info on the TDA729X last night I spent some time reviewing it. Looks like a good choice for me.

[lausar]

Originally Posted by danielwritesbac

I think that I've seen those. It should be fairly easy to replace the single turn trimmer (looks like a dial) with a multi-turn trimmer (looks like a little box with a little screw on it), same for both rails. Then you can precision adjust the rails to identical voltage.

Any mixed/direct coupled amplifiers (amplifies DC by the gain factor) could have a rather bad accident if one power supply powers up before the other in a split rail SMPS scheme (really gigantic offset voltage results). Actually, we should probably consider a reliable center tap transformer and ordinary (or CRC) linear power supply for the amplifier on this thread.


P.S.
My TDA729X projects will easily tolerate SMPS.

I've made up my mind...going with the TDA7296.

[danielwritesbac]

Originally Posted by lausar

I've made up my mind...going with the TDA7296.

It is a good quality part and the datasheet text is sensible even though the examples are not. I do have some alternative designs published for hi-fi with the TDA729X family at lower gain. The TDA7296 will do low gain and hi-fi simultaneously, which could cause need of a preamp if you want it both loud and good. And the safety AC coupled designs for TDA729X will tolerate a twin SMPS (split rail) power source just fine.

That amp could probably do well for most of your needs, but you're in for a bit of trouble to plug that into a tube/valve system that has the preamp as the box for the voltage amp and the power amp as the box for the current amp, which is logical except that the buffer is in the box that you're about to unplug. Oops! In that case, you'd need to add an external (replacement) buffer before the tube preamp can easily drive a solid state power amp, or else suffer lousy bass.

P.S.
Having said all that, the LM1875 design on this thread, works well with an inexpensive center tap transformer and the resulting amplifier doesn't need a preamp at all. The LM1875 on this thread can do an audibly level frequency response in hi-fi, at low cost, despite using an ordinary laptop computer as the source device. That's a great trick, but if your needs/application is much different, then you might need a different design.

[lausar]

Originally Posted by danielwritesbac

It is a good quality part and the datasheet text is sensible even though the examples are not. I do have some alternative designs published for hi-fi with the TDA729X family at lower gain. The TDA7296 will do low gain and hi-fi simultaneously, which could cause need of a preamp if you want it both loud and good. And the safety AC coupled designs for TDA729X will tolerate a twin SMPS (split rail) power source just fine.

That amp could probably do well for most of your needs, but you're in for a bit of trouble to plug that into a tube/valve system that has the preamp as the box for the voltage amp and the power amp as the box for the current amp, which is logical except that the buffer is in the box that you're about to unplug. Oops! In that case, you'd need to add an external (replacement) buffer before the tube preamp can easily drive a solid state power amp, or else suffer lousy bass.

P.S.
Having said all that, the LM1875 design on this thread, works well with an inexpensive center tap transformer and the resulting amplifier doesn't need a preamp at all. The LM1875 on this thread can do an audibly level frequency response in hi-fi, at low cost, despite using an ordinary laptop computer as the source device. That's a great trick, but if your needs/application is much different, then you might need a different design.

I definitely want to use the TDA7296 amp with my tube preamp, no question. Low gain would be great as my speakers are very efficient and a 16ohm load.

[danielwritesbac]

A 16 ohm speaker? A chip amp on 24+24vdc split rail? That's 14 watts max. Is that enough power? Remember, solid state has no cushion of palatable clipping like your tube amp, and therefore the useful/clean power range of solid state is far smaller than max rating. In comparison to a tube amp of 14w, these solid state amplifiers are more like 2.5 watts. The good news is that they'll play 4.5 db louder than your T-amp's 5w max, 0.83w clean.

P.S.
The TDA7265B (higher power version of TDA7265) is an additional competitor for easy hi-fi chip able to run from 24+24v rails. It will appreciate your Twin SMPS split rail regulated power source if NFB cap is used in the amp, and the NFB cap will also increase the dynamics, safety and durability.

[lausar]

Originally Posted by danielwritesbac

A 16 ohm speaker? A chip amp on 24+24vdc split rail? That's 14 watts max. Is that enough power? Remember, solid state has no cushion of palatable clipping like your tube amp, and therefore the useful/clean power range of solid state is far smaller than max rating. In comparison to a tube amp of 14w, these solid state amplifiers are more like 2.5 watts. The good news is that they'll play 4.5 db louder than your T-amp's 5w max, 0.83w clean.

P.S.
The TDA7265B (higher power version of TDA7265) is an additional competitor for easy hi-fi chip able to run from 24+24v rails. It will appreciate your Twin SMPS split rail regulated power source if NFB cap is used in the amp, and the NFB cap will also increase the dynamics, safety and durability.

I'm pushing 3.0 watts now with my SET tube amp and while it is sufficient it's not quite enough power when I want to 'Rock-Out' as they say. The TA2020 amp played loud, louder than my SET amp with all the dynamics I could ask for thou the bass was a bit loose so overall it was enough to make it viable for me. I listened to a Gaincard clone at Horn Club meeting and it was plenty loud, balanced and dynamic on speakers from 95dB to 100dB. Comparable in some respects to the low-powered SET tube amps we were using also. Maybe 30+30vdc split rails will work out better.

[danielwritesbac]

I can't decide for you but I can present options and ask which you favor.

#1
Add a discrete buffer in front of the TA2020 and adjust until the bass is correct. In this case, the goal is to drive the TA2020 correctly. If you can do that, I think you can drive many other solid state power amplifiers correctly. But, if you can't get that done, you might want to limit your power amplifier shopping to those which contain a buffer, except. . .

#2
LM1875 with 100k input load, 110k feedback resistor (220k||220k), 100u NFB cap--4.7K feedback-shunt resistor.
That's one possible "recipe" for "LM1875 Turbo" which is a fixup mod for the K50 kit. I have used that amp before and it has great dynamics. That design tolerates input loads anywhere in the range of 10k to 105.3k. It might not require a buffer, but that is not up to the power amplifier--need of a buffer (or not) is because of whatever source you use to drive the power amplifier.

#3
Building a valve-buffered Gainclone chip amp.
*Can also be done with one 6n3p-DR buffer pushing two of LM4780 ac-coupled parallel amplifiers. Paralleling the Overture chip amps helps to reduce their spike noise and other harmonic distortion that could otherwise cause obnoxiously loud midrange tones. However, a buffered LM4702 is a higher quality approach with labor same or easier than a parallel chip amp--basically the same predrive but with better quality outputs.

[lausar]

Originally Posted by danielwritesbac

I can't decide for you but I can present options and ask which you favor.

#1
Add a discrete buffer in front of the TA2020 and adjust until the bass is correct. In this case, the goal is to drive the TA2020 correctly. If you can do that, I think you can drive many other solid state power amplifiers correctly. But, if you can't get that done, you might want to limit your power amplifier shopping to those which contain a buffer, except. . .

#2
LM1875 with 100k input load, 110k feedback resistor (220k||220k), 100u NFB cap--4.7K feedback-shunt resistor.
That's one possible "recipe" for "LM1875 Turbo" which is a fixup mod for the K50 kit. I have used that amp before and it has great dynamics. That design tolerates input loads anywhere in the range of 10k to 105.3k. It might not require a buffer, but that is not up to the power amplifier--need of a buffer (or not) is because of whatever source you use to drive the power amplifier.

#3
Building a valve-buffered Gainclone chip amp.
*Can also be done with one 6n3p-DR buffer pushing two of LM4780 ac-coupled parallel amplifiers. Paralleling the Overture chip amps helps to reduce their spike noise and other harmonic distortion that could otherwise cause obnoxiously loud midrange tones. However, a buffered LM4702 is a higher quality approach with labor same or easier than a parallel chip amp--basically the same predrive but with better quality outputs.

First off, I really appreciate the time you're making for me on this project. Option #2 strikes me right away as the way I want to go.

[lausar]

I just ordered the K50 kit...

[danielwritesbac]

Originally Posted by lausar

I just ordered the K50 kit...

That is very good news. That amp is fun to work with.

I think you'll need a few supplies from the Radio Shack
A container of Flux--their excellent gel flux can be put on with a toothpick for making very good quality shiny solder connections.
A large variety resistor pack--those (made by Xicon) are quite good.
The 0.022u (22n) and 0.01u (10n) little green polyester to play with for signal bypass caps.
100u for NFB cap (Xicon is not terrible but maybe panasonic is better).

The K50 has one glaring error. The 180k vs 10k-22u feedback setting just has the resistor values up too high for good soundfield. It will sound "clinical" and "flat" in stock condition. This is easy to fix.

You can replace the K50's 180k with a parallel pair of 220k (makes a good 110k) and you can replace the K50's 10k with 4.7k. If both 220k won't "fit down the same holes" you can put the other trackside.
110k feedback with 4.7k feedback-shunt makes the gain 24.4x.
The 22u NFB cap needs replaced with 100u for wide open bass bandwidth. Nearby values work, but the kit's 22u is a bit too small.
After this, you can play with the input load, or in fact you could even use a 100k trimmer (center and one other pin) as a variable resistor to see what input load best suits your tube preamp.

P.S.
With any chip amplifier, I'm sure you'd want to use output caps to protect your rare speakers. With 16 ohm speakers, I'd try a pair of parallel caps for low loss, such as 1500u//1500u and nearby figures work. If you get the size just right for your particular speaker (and room response), output caps works for headroom management and more bass rather than less bass. So, speaker protection can be really fun, especially when it is time to rock the house. Well, that's what I use them for. Trimming off a little bit of subharmonic that the speaker wasn't going to output anyway, saves enough power to make the LM1875 play as loud as a 50 watt amp.