Run 1 amp or two?

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punkmike

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Run 1 amp or two?
« on: 14 Sep 2015, 01:09 pm »
Just got a pair of Polk T15 bokkshelf speakers to go with my self powered Polk 10" sub


My old setup was a Lepai 2020 amp and it was ok but I wanted it more powerful.


Can I run a Lepai 2020 amp  to "each" of my new Polk T15's?


...or better to just buy 1 amplifier to power them?

FullRangeMan

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Re: Run 1 amp or two?
« Reply #1 on: 14 Sep 2015, 01:37 pm »
Do you mean say bridge the amp? 2 chan into 1.
Some amps are not bridgeable, according the output circuit.
The Musical Fidelity E5 I burn are not bridgeable.

Look the at the Owner Manual for it or email manufacturer
for a safe reliable opinion.

richidoo

Re: Run 1 amp or two?
« Reply #2 on: 14 Sep 2015, 01:54 pm »
Hey Mike!

Some links:
http://www.lepai.us/amplifier-lepai-lp-2020a.html
http://www.kafka.elektroda.eu/pdf/tripath/TA2020.pdf
http://www.amazon.com/gp/product/B002RJLHB8

Polk recommends 20-100W per speaker. Usually you want to lean towards the top end for the best performance, if possible.

TA2020 chip used in the Lepai amp makes max 18 watts at 4ohms, and 10W@8 ohms, but thats at 10% THD, which is often how chip amps originally designed for car stereos are rated. At 10% distortion the amp is right on the edge of clipping into a perfect resistive test load. Into a reactive speaker load with some negative electrical phase you will get slightly less power at clipping. Any amp playing near clipping sounds horrible, unlistenable by our standards. It may also have built in self-protection system which shuts the amp off when it senses a threat. Near clipping there will be thermal events that engage the protection which sounds even worse than voltage clipping, so using a chip amp at full rated power is not really possible.

For stereo listening with good speakers you want to focus on the 0.1% power rating. For this amp, playing in a car running on the highway with alternator providing 14.6V the 0.1% power output is 11W@4Ohms, and 7W@8Ohms. Using a 12V wall wart power supply would be a little less power.

The Polk T15 has a nominal impedance of 8ohm. While probably dips below that at some frequencies, it won't dip too far, because it is an inexpensive speaker that will most typically used with inexpensive HT receivers which don't perform well with low impedance speakers. You might be able to find an impedance graph of the actual load this speaker offers somewhere on the web, or by writing to Polk. But it's safe to assume it is a 8 ohm speaker, and that you have about 7 watts per channel from the Lepai. 

Since Polk recommends 20W minimum per channel, I'd say you are underpowered with the Lepai delivering ~7W.

Typical of car stereo chips, this chip actually has 4 amps inside which are already bridged internally so additional external bridging of the stereo channels to get more power is not possible. And since the T15 has only one set of inputs, it is not bi-ampable. So there isn't any way of supplying it with 2 channels worth of power to each speaker (if that's what you were asking?)

You can find a heap of used receivers on Craigslist that will provide 50W/ch for a very low price. They get tossed because they don't have HDMI, but the audio section is still good. Then you might get a remote and radio too.
Good luck
Rich

JLM

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Re: Run 1 amp or two?
« Reply #3 on: 14 Sep 2015, 02:17 pm »
Well, the Polk T15 is rated to handle 100 watts per channel, so I'd try wiring the amp you have to one of the speakers to see if/how it works.  (You'll need a "Y" connector to split one incoming right or left channel from your source to feed into both channels of the amp.  And you'll need to fit both pairs of speaker wires onto the posts on the backside of the speaker.)

Keep in mind that doubling the power only adds 3 dB of volume (sound pressure level) which translates to a "half again" louder sound.  (Twice as loud would require a ten-fold increase of power, which the T15 isn't rated for.)

The other consideration is trying to match the volume on each amp.  Being an inexpensive piece, the controls probably aren't very precise (the volume level when set at 10:00 on one may not correspond to 10:00 setting on the other amp).  You'd just have to learn to match by ear the two controls.

Note that speaker efficiency can make more difference with volume than amp size.  Examples: 

83 dB/w/m speakers with 30 watts per channel = 97 dB maximum
89 dB/w/m speakers with 7 watts per channel = 97 dB maximum  (This is the efficiency of the Polk T15, a fairly average rating.)
94 dB/w/m speakers with 2 watts per channel = 97 dB maximum

This assumes you're comparing apples to apples.  Audio manufacturers rarely use the same standards to measure by, so "real world" performance can vary widely and as richidoo states the Tripath has a useful rating of 7 watts per channel (I owned a nice one years ago).


Better to buy 1 amplifier?  Well, it would save buying "Y" interconnect splitters and fussing with two volume controls.  It would also allow you to move up to a nicer sounding/significantly more powerful amp.  If you're into DIY the TPA 3116 would fit the bill very nicely.