No bass in 300b amp

[ebuckley002]

Apologies if this has already been covered, but I am pretty stuck with a 300b amp design.

I'm building the JE Labs SE variant(SE300B classic and DX) and mostly things are fine, except that the output is very quiet and has hardly any bass at all.

Does anyone have any ideas what might be wrong?

[rknize]

Some more details about your speakers, their sensitivity, your output transformers, etc might bring more specific feedback.

[ebuckley002]

I'm using Polk Monitor 50's (quoted sensitivity 89dB), o/p transformer Hammond 1627SEA.

Hope that's specific enough

[kevinkr]

Schematic showing your version of the build (with any changes you made) would also be helpful.

IMHO the efficiency of those speakers is a mite on the low side for a 300B amp.

[Glowbug]

And who knows if that's a "proper" number, 1w/1m, or 2.83V...or how complex the Xover is.

[ChrisA]

Originally Posted by ebuckley002

I'm using Polk Monitor 50's (quoted sensitivity 89dB), o/p transformer Hammond 1627SEA.

Hope that's specific enough

I use those same speakers connected to a Yamaha digital piano and powered with and Audio Source AMP100. This is not for music reproduction, in this case it is a musical instrument used for practise.

The AMP100 does 50 WPC into 8ohm and with the M50 speakers I need to turn the volume control all the way up. But the bass is quite good. Because the system is conected to a piano I know exactly what frequenccies I send, (middle C is aboutr 260Hz, the next C down is 130Hz and so on.) the speaker does 50Hz no problem but I sit on the piano bench only 3 feet from the speaker and the volume on a 50W amp is at "10".

For music reproduction in a decent sized room I'd recommend about 100WPC for those speakers unless you like to play at low lovels then 50WPC is OK.

I suspect your amp is very much under-powered for those speakers. If they are spec'd for 89 dB then that is likely measured at 1KHz or something like that.

A better way to test the amp is to get an 8 ohm dummy load and measure the AC voltage across the load while sweeping a test signal in the amp's input.

[wboyd]

I used to own a pair of those (back in my HT days)...and I am pretty sure that a minimum of 20 watts is required...I think the range was about 20-150?

I am not sure what the output on that particular amp is, but before you start making major changes and If you have the ability to, try some more efficient speakers - maybe a pair of full rangers.

I am currently running a little spud amp (6T9 tubes at about 1.5-2 watts) with some fostek FE103E's (efficient to about 98db) that puts out a sweet sound and is very "room filling".

[vinylkid58]

Originally Posted by wboyd

I am currently running a little spud amp (6T9 tubes at about 1.5-2 watts) with some Fostex FE103E's (efficient to about 98db)

Spec sheet says 89dB, or am I missing something?

Jeff

[wboyd]

opps...my mistake...

Forget to mention that I am using high effiency BL Horns...

[Eli Duttman]

Paul Joppa's 102 dB. rule tells us the minimum efficiency to use with the 8 WPC SE 300Bs yield is 93 dB. Clearly, the general lack of listening volume is directly related to insufficient power O/P.

I suspect the poor bass performance is related to 3 factors. The damping factor of a no NFB SET amp is inadequate to control the woofers in that MTM speaker design. The 2nd factor is the comparatively low primary inductance/impedance of the 1627SEA O/P trafos. IMO, the 1630SEA would have been a better choice. The 3rd factor is the bass extension of the speakers themselves, which is 3 dB. down at 57 Hz.

While little can be done about the O/P power level, a quick experiment is available to test the damping factor idea. Connect those nominal 8 Ω speakers to the 4 Ω taps.