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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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 Post subject: LM386 head amp?
PostPosted: 04 Oct 2011, 21:58 
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Joined: 28 Dec 2010, 22:07
Posts: 265
Hi all,

I am working on a project that is pretty much runoffgroove.com's Ruby amp. I'm not intending on using the extra gain of the chip itself (pins 1 and 8), instead using a BJT stage in front of the MPF102 buffer to provide a little overdrive. I may even give diode clipping if I feel froggy.

I am building it in a metal chassis that is intended to look like a high-quality head amp. I even have thought of getting a couple of dead EL34 or something and mounting them to the chassis with the chips inside (8 pins, right??). Although it will be low-power, it will at least look better than the Ruby amps built inside cereal boxes or beer cans.

I have a simple question. Will paralleling several LM386s help cool the chips by sharing the dissipation between them? I know that, given an infinite amount of chips, I'll only get so much out. The more chips I use in the output stage will (I think) spread the heat and cook each one less. With each chip the input Z decreases but the power won't go up that much more. I don't want to try driving a 4-ohm cabinet to a watt with just one chip.

Am I wrong or am I on to something?

Ed

_________________
Completed Projects:

6L6G / 12AX7 25W Guitar Amplifier
TLC272CP/2N3904/3906 Headphone Amp (Can Man)
MPF102/IRF630 Headphone Amp (FETZilla)
12AU7 / IRFX Headphone Amp (x2)
TIP31C/32C 5W Stereo
More to come...

Bow to Ed!!


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 Post subject: Re: LM386 head amp?
PostPosted: 04 Oct 2011, 23:42 
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Joined: 06 Apr 2009, 10:08
Posts: 841
Location: US Pacific Northwest
A couple of issues to address here.

First, parallel LM386s is not the best idea. If you parallel before the output cap then the imbalances between the chips will drive the bias points and one or both chips will have unbalanced action. Think HEAVY 3rd harmonic distortion. Regardless of before or after the output cap, the feedback between stages will be unbalanced and you'll likely get oscillation. Even if it does comes out stable, the distortion will be uncorrected by the feed back and very high.

Second, the N package (DIP) is rated for 1.25W with a 107ºC/W JA thermal resistance. You'll have to do the calculations to see how far you can drive dissipation in your assumed operating temperature. A DIP heat sink may be a good solution. However, regardless of this, the 386 is only capable of at best 350mW into a 4Ω load. It will go over a watt into a 16Ω load which is why the 386 makes such a fine headphone amp (according to some, not to others. Read this for my experience, 4th paragraph down.). But attempting to get a watt into 4Ω you'd be better to use the 386 to drive a Darlington pair biased for class A operation. Or you could try a different chip more suited to higher output.

The bottom line is that the LM386 was always intended as a small, low power solution for portable radios, small consumer goods, and the like. Personally, if would suggest that if you're going to go to the effort to build something that looks half way decent, you should invest in a chip more up to the task at hand. IMHO.

I've used the 386 for a long time for all kinds of projects just because it's a simple one chip, no thought solution to providing an audio output on a circuit. But as a dedicated amp, I have always found it somewhat lacking.

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 Post subject: Re: LM386 head amp?
PostPosted: 05 Oct 2011, 02:50 
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Joined: 06 Jun 2008, 18:23
Posts: 3610
Location: Australia
Not sure if Matt said this but use the 386 to drive an output stage. I single suitable transistor or FET will give you an Class A output stage (if biased correctly) with much better drive capability.

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Projects: "Abstinence" - 6EM7 power amp | "Sanguine" - 6AS7 Headphone amp with Sophia Mesh Plate 274B rec. tube | "VoXDeuX65" - Inexpensive 2-Way Speakers ]
Website: [ retro-thermionic ]


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 Post subject: Re: LM386 head amp?
PostPosted: 05 Oct 2011, 07:41 
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Joined: 28 Dec 2010, 22:07
Posts: 265
I appreciate the advice. I still intend to try it on the breadboard and see what happens. I figure that 0.35W into 4 ohms is close to 300 mA. If I can get four chips to source 150 mA each I can get 1.44W. This probably won't happen but I can try. The small chip was running warm to hot when providing a pretty loud volume in my cabinet, so four chips sharing a slightly heavier amount of current should be able to do so without getting overloaded. Should; that's why I am testing on my breadboard.

Some research seems to indicate that a resistor in the output lead of each chip will help stabilize the gains and prevent the operating points from fighting one another. I intend to use a 1 ohm resistor per chip to see if this is true. This project isn't about finding the absolute best part for the job but finding what I have and getting the most out of it. In May I built a 25W guitar amp with 6L6Gs and three 12AX7s. That is my power house, one I won't turn up to full because of my apartment-dwelling status.

I appreciate the input. I built a pair of headphone amps this year using a TLC272 and a totem pole 2N4401-2N4403 combination that sounds good. Strangely, a TLC072 or 082 can't drive the transistor pair like the 272 can. Many say that the chip I chose is awful for audio but I can't hear the difference. If they want to go out and buy $15 op amps, by all means...

Ed

_________________
Completed Projects:

6L6G / 12AX7 25W Guitar Amplifier
TLC272CP/2N3904/3906 Headphone Amp (Can Man)
MPF102/IRF630 Headphone Amp (FETZilla)
12AU7 / IRFX Headphone Amp (x2)
TIP31C/32C 5W Stereo
More to come...

Bow to Ed!!


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 Post subject: Re: LM386 head amp?
PostPosted: 31 Oct 2011, 09:24 
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Joined: 02 Aug 2011, 12:38
Posts: 5
Location: Portugal
check http://www.beavisaudio.com/library/LM386/LM386.htm for a simple 2xlm386 bridge config.


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 Post subject: Re: LM386 head amp?
PostPosted: 31 Oct 2011, 18:12 
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Joined: 06 Jun 2008, 18:23
Posts: 3610
Location: Australia
Have to say I'm real impressed by the mini amp. Both the sound and finish of the small boxes. Very, very impressive.

_________________
Projects: "Abstinence" - 6EM7 power amp | "Sanguine" - 6AS7 Headphone amp with Sophia Mesh Plate 274B rec. tube | "VoXDeuX65" - Inexpensive 2-Way Speakers ]
Website: [ retro-thermionic ]


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 Post subject: Re: LM386 head amp?
PostPosted: 01 Nov 2011, 08:00 
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Joined: 28 Dec 2010, 22:07
Posts: 265
The original plan was for a guitar amp with several output devices in parallel. There's nothing wrong with bridging LM386 but I'm thinking it's kind of worthless to put so much effort into such low power. I have some PSU mosfets that, although would be ill-suited to go in an audio amp, would do well in a guitar amp which doesn't need to go all the way up to 20 KHz.

Being that I only have one, I could make a Zen-like class A amplifier with a 10W output. The drawback is the amount of current needed for such an output. If I used two of the same N-channel fet, I could get away with using a transistor phase splitter in the input. It isn't impossible to get P channel mosfets to match the N-channels but it is somewhat tough. I'm sure there is a database somewhere that outlines what devices match.

Ed

_________________
Completed Projects:

6L6G / 12AX7 25W Guitar Amplifier
TLC272CP/2N3904/3906 Headphone Amp (Can Man)
MPF102/IRF630 Headphone Amp (FETZilla)
12AU7 / IRFX Headphone Amp (x2)
TIP31C/32C 5W Stereo
More to come...

Bow to Ed!!


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 Post subject: Re: LM386 head amp?
PostPosted: 01 Nov 2011, 16:04 
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Joined: 06 Jun 2008, 18:23
Posts: 3610
Location: Australia
For a few dollars you can buy a TDA1905. This is a 5W chip with low power requirements etc. When driven hard it will crank 10% distortion but for guitar amp that should be fine. It may need a preamp stage to get the 100mV from the pickup upto a little more to get full 5W out of the chip. Or use a two stage tube front-end with a 12AX7 from the Fender Champ 5F1 schematic (built one, great amp). This will give a little tube warmth and a SS 5W output. But now we are talking AC plug packs to power the unit.

Without the tube front-end the amp should run on a few 9V in series and parallel. 18V appears to be a good voltage level for the chip.

_________________
Projects: "Abstinence" - 6EM7 power amp | "Sanguine" - 6AS7 Headphone amp with Sophia Mesh Plate 274B rec. tube | "VoXDeuX65" - Inexpensive 2-Way Speakers ]
Website: [ retro-thermionic ]


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 Post subject: Re: LM386 head amp?
PostPosted: 01 Nov 2011, 16:58 
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Joined: 28 Dec 2010, 22:07
Posts: 265
I have decided to nix the idea of using LM386s or any chip amplifier. I like using discrete designs much better because it gives me something to design. I think I was just messing with the 386 idea because I was looking for an easy answer. The 386s I have might end up on Craigslist or eBay as a bundle. I have quite a few and some are the JRC brand.

I don't mind putting in an AC plug pack. I was originally going to put in a transformer and power supply. The extra work is worth it to me just so I never have to buy batteries. A laptop charger will give me a fairly high voltage which I can filter down to 12-15 and run an amp from. 12 volts can give me a couple of watts and that will be loud enough for what I hope to accomplish. I don't perform in large auditoriums so I will likely never need a 100W amp. That won't stop me from building one using EL34 or some strange FET I scavenge from a power supply. Who knows.

I have a few TDA2050 which sound like a similar chip. They are the TO-220 type case with 5 pins. It doesn't sound bad but the sound fails to impress when compared to a genuine tube amp. I do have a 25-watter made with 6L6s and I'm perfectly happy with that one. I'm more about experimentation right now.

Ed

_________________
Completed Projects:

6L6G / 12AX7 25W Guitar Amplifier
TLC272CP/2N3904/3906 Headphone Amp (Can Man)
MPF102/IRF630 Headphone Amp (FETZilla)
12AU7 / IRFX Headphone Amp (x2)
TIP31C/32C 5W Stereo
More to come...

Bow to Ed!!


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 Post subject: Re: LM386 head amp?
PostPosted: 03 Nov 2011, 23:55 
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Joined: 28 Dec 2010, 22:07
Posts: 265
I think I know what I am going to do. The reason I want this small guitar amp is not for a unique, beautiful tone that will inspire youngins to grab an axe and head to the Church of Saint Van Halen. It's intended to be somewhat clean although a little breakup is okay. I intend to have a small, cheap amplifier with which to evaluate speaker cabinets and effects that I build. To that end I am not going to go all-out on the transistor output stage of the century.

The TDA2050 is a bad-ass power chip with a maximum power voltage of 50 (or +/- 25) and a max power of 32W. I will be running it off of a 12-18v supply so it's not going to get much more than a couple of watts (five would be more than enough). This chip with its extreme limits will provide more than a DIP-8 on which I'm pushing the edge.

TDA2050 is wired more like a conventional op amp than the 386 so the circuit will have to be completely redesigned. I lap-soldered some wires to the pins and will be breadboarding with it in the coming days, given I get some time. The freedom is that I don't need to use a 1K log pot for the gain, giving me some options among parts I already have. Also, being that the input impedance for the chip is 500K, I don't explicitly need to make a JFET input buffer. I could anyway, increasing my input Z by at least three-fold.

The one drawback I see with the chip is that it has a minimum gain of 24 dB. I will need to wire the amp so the gain never drops lower than that. With a 12v supply the output will never be more than a few watts. That's all I need or expect from this amp. The extra headroom this chip offers will allow me to get more clean volume, effectively letting me know that the distortion comes from the effect and not the amp itself.

Ed

_________________
Completed Projects:

6L6G / 12AX7 25W Guitar Amplifier
TLC272CP/2N3904/3906 Headphone Amp (Can Man)
MPF102/IRF630 Headphone Amp (FETZilla)
12AU7 / IRFX Headphone Amp (x2)
TIP31C/32C 5W Stereo
More to come...

Bow to Ed!!


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