Simple Chip Amp for P to P wiring

[coldcathode]

Hello All,

I spend most of my time over on the tube forum, SS just has not been my thing. I am in the process of designing my new reference system.
Details have not been ironed out but the system will rough out like this.

Tube Phono Preamp, CD, PC sound, Tuner into

Tube preamp w/ active 100hz TUBE crossover 12db/octave Linkwitz alignment.
The highpass will be split with another tube active Xover and biamped via tube amps to speakers to be determined.

The low pass is where my question comes in.

Most of my budget is blown on tube amp iron, tubes etc. So I am limited on the budget for the subs and amps. This mean relatively low sensitivity woofers in sealed enclosures. The power needed to drive these will be well beyond SET and even most PP Pentode tube amps so I want to try my hand at a chip amp.

I am looking for about 60WPC with a simple gain control nothing else. I would like to wire it Point to Point in keeping with the theme of the rest of the system.

So what I am looking for is a simple chip amp schematic that would lend itself to P to P wiring and with cost as the major concern.

[ted4412wilt]

"$35 each. This product is exempt from the $80 minimum order rule, when shipped to an address in the US, due to the fact that it is already packaged to ship as is..

Foster subwoofer plate amplifier. 115W into 4ohms, 80W into 8ohms. 4th order (24dB/octave) low pass adjustable from 50-200Hz. 0-180 degree phase switch, RCA and speaker level inputs, volume control. Unit has a 3rd order (18dB/octave) high pass filter at 29Hz with a Q of 0.707 built into the subwoofer section. Can easily be modified for any Q and cutoff frequency that you want by changing R615, R617, R618, C607, C608 and C609. The amplifier also has a spectral tilt control that is adjusted to give about 10dB of shelving. It can be manually adjusted with a pot on the PCB. See response curves linked below. A drawing of the amplifier and schematic are both linked below. Two of these amplifiers can be bridged together to produce 220W into 8ohms. See the FAQ below for details of bridging and filter modifications."
NHT drivers and electronics surplus sale
You cannot build anything this powerful for $35.
Ted

[wintermute]

Hi coldcathode, I did a P2P LM3886 using the schematic in BrianGT's Chipamps.com kit. Though you will only get 60W if you are driving a 4 ohm load.. if you are driving 8 Ohms you can safely get 50W.

Tony.

edit: I think I spent around $130 AU on the amp... the Transformer was the most expensive part.

[ted4412wilt]

http://dogbreath.de/Chipamps/CopperAmp/CopperAmp.html
http://dogbreath.de/Chipamps/ThreeResAmp/ThreeResAmp.html
Mick Feuerbacher Audio Projects has some excellent step by step instructions with pictures of 3886 point to point amplifier. You should look at Linkwitz website for his version of 3886 too which includes safer input circuit.
http://www.linkwitzlab.com/images/graphics/3886pwr.gif
Ted

[coldcathode]

ted,

AWESOME links, EXACTLY what I was looking for!!!

I in no way want to belittle solid state chip amps but I really do not want to spend much time & effort on these amps. Can you or someone else point out for me all the basic info needed to build one of these?

For instance, would 100VA trannys be enough? example 25V @ 4 amp? Bridge rectified I am looking at about 35 Volts?

How can I calculate the power requirements assuming a 4ohm load? I can design tube amps all day long but I need some basics on these amps.

In the schematic for the copperamp I have a couple questions, if you can answer them for me it would be great. (I am willing to assist SS guys looking to get into tubes)

R1 (pot) is used only for volume and sets input inpedance to the source correct? Does it have any effect other than attentuating the signal?

Rg what is Rg use for? some sort of input bias?

Rf gives feedback? how is that value calculated in relation to load power etc.

Rm is a muting resistor and is switch in and out I suppose?

Cs The caps are for smoothing the supply and "headroom" I suppose, doe their value have any significance? ie can I raise the value for more "headroom" considering relatively small trannies?

[ted4412wilt]

Use 18-0-18 toroid with at least 160 VA rating. 25 volt AC gives too high DC for 4 ohm speakers and will make too much heat on the 3886. I am looking for link for Antek toroids but it seems to have changed. They were one of the best toroid suppliers.

Sorry I do not have enough time and knowledge to cover all of your questions. Again I will remind you that it is not cost effective to build an amplifier.
Ted

[coldcathode]

Ted,

Thanks but it is also NOT cost effective to build TUBE amps but it's what I do. I guess what I am looking for is a "primer" on chip amps.

I will browse thru this forum and see what info I can gather.

[ted4412wilt]

Don't overlook class D or T amps for reduced power supply and heat sink cost because of high efficiency. I bought 4 stereo 9 watt per chanel .04% HD at 4 ohm boards that I run from an old ATX power supply. Total deIivered cost was $41. I need several low impedace amplifiers to use for a system using many drivers which I do not want to run in series. Several efficient powered speakers can produce plenty of low distortion sound. I already have two powered subwoofers. Good luck in your adventure. Someday I may go insane and build a high power very low impedance class D amp with digital input.
Ted

[wintermute]

Hi ColdCathode,

I'd highly recommend reading at least pages 18-21 of the LM3886 datasheet Page 17 might also be helpfull if you are planning on using the smallest heatsink you can get away with.

Rg I'm unsure, I'm even unsure as to whether I had one of these on mine. I mostly used BrianGT's schematic but also took queues off the typical application in the datasheet (which doesn't have this resistor).

The datasheet has the info on calculating the feedback resistors (It is the ratio between rf and ri that sets the gain).

rm provides the necessary current to turn off the muting fuction (ie it will be muted if left out). If you don't want a mute function simply hard wire the resistor in

Cs is a supply bypassing cap. I will quote the following from the datasheet:

SUPPLY BYPASSING
The LM3886 has excellent power supply rejection and does not require a regulated supply. However, to eliminate possible oscillations all op amps and power op amps should have their supply leads bypassed with low-inductance capacitors having short leads and located close to the package terminals. Inadequate power supply bypassing will manifest itself by a low frequency oscillation known as motorboating” or by high frequency instabilities. These instabilities can be eliminated through multiple bypassing utilizing a large tantalum or electrolytic capacitor (10 μF or larger) which is used to absorb low frequency variations and a small ceramic capacitor (0.1 μF) to prevent any high frequency feedback through the power supply lines. If adequate bypassing is not provided the current in the supply leads which is a rectified component of the load current may be fed back into internal circuitry. This signal causes low distortion at high frequencies requiring that the supplies be bypassed at the package terminals with an electrolytic capacitor of 470 μF or more.

You can use bigger caps in the power-supply which I would say would be a good idea if you are using this for bass duty. A lot of people claim that the LM3886 is better with smaller capacitance in the powersuply, but I think usually the reason is because of perceived differences in the midrange and higher freqs, so for bass duty I think would be worth it. Also I would not recommend using ceramics as per the app note quote above, but instead some good quality film capacitors. I used polyester in mine, but I would consider using polypropylene or pps types now.

Tony.

[jb74]

ted4412wilt

I have dealt with Antek located at -

http://www.antekinc.com/

jb74