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Chip Amps Amplifiers based on integrated circuits

LM1875 or TDA7498 For good clean sound (Boards shown in the post)
LM1875 or TDA7498 For good clean sound (Boards shown in the post)
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Old 30th April 2018, 08:07 AM   #161
Dunk02 is offline Dunk02  Australia
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Quote:
Originally Posted by FauxFrench View Post
Fair enough, then most will qualify because the datasheets normally give quite wide tolerances.
Some only find the use of high quality components as designed correctly and then the definition becomes subjective and more difficult to handle.
I quite like objective measurements myself and cannot hear some high frequencies. With an A/B comparison rig I don’t think I could easily hear any differences. To be honest I don’t have an appreciation of the audiophile qualities often expressed for hifi gear.

I do like playing with these boards though, and I do try and differentiate their particular sound qualities, mostly without much success. All sound pretty good to me within their clean power ranges. I had read that the Yamaha chips produced outputs with higher sibilence - I couldn’t hear it!

Anyway, just wondered if there was a preferred chinese board these days for the LM1875 chip. As an example here is one that claims all types of improvements.

CG version LM1875 lower distortion and more resistant version amplifier board kit-in Operational Amplifier Chips from Consumer Electronics on Aliexpress.com | Alibaba Group

Last edited by Dunk02; 30th April 2018 at 08:18 AM.
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Old 30th April 2018, 06:06 PM   #162
FauxFrench is online now FauxFrench  France
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Quote:
Originally Posted by Dunk02 View Post
I quite like objective measurements myself and cannot hear some high frequencies. With an A/B comparison rig I don’t think I could easily hear any differences. To be honest I don’t have an appreciation of the audiophile qualities often expressed for hifi gear.

I do like playing with these boards though, and I do try and differentiate their particular sound qualities, mostly without much success. All sound pretty good to me within their clean power ranges. I had read that the Yamaha chips produced outputs with higher sibilence - I couldn’t hear it!

Anyway, just wondered if there was a preferred chinese board these days for the LM1875 chip. As an example here is one that claims all types of improvements.

CG version LM1875 lower distortion and more resistant version amplifier board kit-in Operational Amplifier Chips from Consumer Electronics on Aliexpress.com | Alibaba Group
I am convinced you will appreciate that board. For France I would have to double the price when including sending costs. Australia is much more moderate.
What I see from the photos are good buffer capacitors, audio-type capacitors probably used as coupling capacitors, Omron speaker protection relay, well sized Thiele-filter coils, good capacity screw-terminals. All-in-all, I cannot point at any weaknesses. If the PCB layout is optimal? Probably it's OK, also the Asians do rarely mess it up severely. And, for that price you risk little.
I am certain it will perform to datasheet specs.

I only have experience with the Yamaha YDA-138E and that sounds great to me (considering its limited power). With my age I probably have an important roll-off above 12KHz. That's life.

Last edited by FauxFrench; 30th April 2018 at 06:09 PM.
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Old 30th April 2018, 09:12 PM   #163
Dunk02 is offline Dunk02  Australia
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Thanks very much for taking a look at that board and providing some feedback on the layout.

An 80VA toridal transfirmer would suit this ?
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Old 30th April 2018, 09:54 PM   #164
FauxFrench is online now FauxFrench  France
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A 80VA transformer is a bit marginal.

Your can choose between 2x15V and 2x18V. Rectified that should leave
2x23Vdc(idle) and 2x28Vdc(idle). Max. for LM1875 is 2x30V. Take the 2x15V unless you want all the LM1875 can give.

The LM1875 is current limited at 4A peak, that means 2.8Arms.
An amplifier operated from a symmetrical supply draws current from only one of the rails at a time. Thus, for two channels we need 2x2.8Arms, but we can halve that since each amplifier only draws current from one rail. Therefore 2.8 Arms.

80VA/2x15Vrms = 2.7Arms
80VA/2x18Vrms = 2.2Arms

As an Australian I guess you will be playing BeeGees, Olivia Newton John or AC/DC!. Why is that important: because of the crest-factor. Thus, the variation in sound intensity. For music with very varying intensity (acoustic guitar) the crest factor is stated by clever forum members to be around 4. For music with a permanent intensity (AC/DC) assume perhaps 2.
But, you draw the current from a rectified voltage that is higher than the ACrms voltage for which reason the rms current rating may be reduced.

Conclusion: For the traditional "permanent output" (DIN-norm) output power you should rather have a 120VA transformer. If you include the crest-factor you can get away with 80VA, even with AC/DC.

My advise is, if you already have an 80VA use that one. If you have to buy anyway, go for 120VA. It need not be a toroidal transformer but they are the easiest to find nowadays.

Last edited by FauxFrench; 30th April 2018 at 09:57 PM.
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Old 1st May 2018, 07:10 AM   #165
Dunk02 is offline Dunk02  Australia
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Excellent explanation - the advice is much appreciated. Your references to Ozzy music made me laugh!

Are toroidal transformers with shielding between the primary and seconday windings necessary, or a prudent buy? Any suggestions regarding the placement of the transformer; inside the amplfier case vs outside?

On eBay.com.au at the moment I see:
120VA 18V $55
100VA 15V $50
I would have gone for the 18V unit but our supply is actually 240V and with any spikes I guess this could exeed the 30V limit. So the cheaper one is probably the one to get?

eBay listings for heatisnks don’t generally have C/W ratings. Any ideas on approximate physical size for a passive heatsink? I have a couple of heatsinks measuring 150x60x25mm.

If this board had a gain of 10 (according to the TI spec sheet, not sure what it is exactly), what gain range would I need in my preamp if the source was headphone jack or line level (I think the Apple Airport Express is this)? I have an NE5532 preamp board or an NE5532 preamp board with tone control with gains of 5 or 10 to use for this purpose (I have been gathering resources��).

Thanks

Last edited by Dunk02; 1st May 2018 at 07:16 AM.
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Old 1st May 2018, 08:51 PM   #166
FauxFrench is online now FauxFrench  France
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If we use the current limit of 4A as a starting point, we can calculate:

4A in 4Ohm requires 16V. (2x15V transformer is enough to use the LM1875 to its full capacity).

4A in 6 Ohm requires 24V. (2x15V transformer is not far from what is required to use the LM1875 to its full capacity).

4A in 8 Ohm requires 32V. (2x15V transformer is not enough to use the LM1875 to its full capacity; 2x18V may be a better choice).

So, I can suggest: If you use 4 Ohm speakers, go for the 2x15V transformer. If you use 6 or 8 Ohm speakers, consider the 2x18V transformer.

240V/230V is some 4% more. About a Volt at the secondary. It should not exceed 30 Volt. As a trick if you are just above 30V, use double rectifier diodes before the storage capacitors (evidently). That leaves you with a bit of extra voltage drop and the activation of the diodes and the transformer winding will be less abrupt -> less rectification noise.

On the shielding between primary and secondary, my feeling is:
If it costs very little, take it along. I do not think, any of my transformers have got such shielding inside. IF you have a very noisy input from the net or your load on the secondary produces exceptional noise you do not want coupled to the net, it may be useful.
I use these EMI filters you find included behind the female connector for a standard net-cord. I believe I need nothing more.

Transformers have a leakage field that may cause hum in the amplifier. On the other hand, the transformer may be shielded from the amplifier with an iron plate. Toroid transformers have less leakage field. I have seen many well functioning amplifiers with the transformer and amplifier in the same casing and even without shielding.
I theory, the more distance - the better (with a limit of course).

It sounds to me that one 150x60x25mm heatsink may be sufficient. Arrange it so that you have a good airflow around it. Unless you realize it clearly isn't sufficient, I wouldn't buy another one for a start. For normal use you rarely get above 2x5W output power.

Ozzy music is something I wasn't aware of for long. I am raised in a rather Anglo-Saxon tradition and corresponding humor. It includes John Cleese like over-exaggerated prejudice: evidently Ozzies play Ozzy music. I couldn't help including that prejudice in my posting. During my whole youth we listened to songs mainly with English texts. Most had a British origin though some were believed to be American (no Internet in those days). Only more recently I realized that many good songs and singers had Ozzy background. One thing about Ozzies, they look very much like anybody else even though they live all the way "down under". Often they are more pleasantly relaxed (must be the sun) than us Europeans and they tend to use the word "mate".

Last edited by FauxFrench; 1st May 2018 at 09:20 PM.
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Old 2nd May 2018, 10:10 AM   #167
Dunk02 is offline Dunk02  Australia
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Thanks again for the info. What I need is a multi-tap transformer and a switch on the amp as I have several pairs of speakers. Likely I will mostly use 8ohm speakers though.

I am only partially Ozzie and have to confess to having a mostly “Roast Beef” childhood so probably have similar exposure to British music and comedy as yourself.

The new amp will mostly be playing early Fleetwood Mac, Dire Straits and Pink Floyd.

I can also admit to having some French cousins! And I once broke done in Laval (if I remember the town’s name correctly) and had to rely on my high school french to get my car repaired - it was a memorable trip in 1999!
Thanks again!
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