VP8NO writes: > Perhaps someone suitably qualified and tooled up might like to try and > [repeat] and comment upon the view presented by Rob Sherwood at the > Dayton convention. According to him the K3 audio amp stage has > serious shortcomings.
I had to listen to the audio of Sherwood's talk before looking in the right area. First, Rob doesn't say the K3 audio has serious shortcomings, he complains about distortion on the speaker. Sherwood also admits that using a high impedance speaker (e.g., amplified computer speakers) or high impedance headphones resolves the issue! My measurements did not show that distortion but I was measuring at the headphone output with a high impedance load - the lowest impedance headphones I use are 32 Ohms. As Jack Smith points out, the headphones and the speakers have separate audio amplifiers but share a common DAC. Since Sherwood's comments were based on the speaker output and a LOW impedance load, I dragged out an old Optimus XTX25 speaker - an unpowered 8 Ohm speaker I've used with other radios for many years. With the 8 Ohm speaker is was easy to generate harmonic distortion - particularly if the K3 were set to SPKRS = 2 with only one speaker connected - simply by turning up the volume until the amplifier was forced into clipping/saturation! With the 8 Ohm speaker, 1 to 2V peak represents a comfortable to loud listening level for me. At those levels, the harmonics are all down more than 65 dB and any other distortion is very acceptable. The following table shows harmonic levels relative to a 500 Hz reference tone at the given voltage across the speaker. 0 dB is 1V Peak (.707V RMS). +10 dB represents 2.25V RMS (signal + distortion). +13 dB is 3.2V RMS (the speaker amplifier is in the compression region at the threshold of clipping) and +15 dB is 4V RMS (the speaker amplifier is in hard clipping). Reference 2nd 3rd 4th 5th 6th 7th 8th 9th ============================================================ +15 dB -37 -15 -45 -26 -60 -43 -51 -37 +13 dB -49 -31 -51 -35 -57 -39 -63 -49 +10 dB -70 -68 -95 -67 -85 -69 -85 -73 0 dB -66 -74 -70 -77 -73 -76 -77 -80 -10 dB -54 -66 -63 -90 -73 -82 -80 -87 -20 dB -50 -78 -68 -74 -73 -74 -74 -78 In order to drive the audio amplifier hard enough to create the distortion, it was necessary to disable the AGC or set AGC SLP to low values (reduce the AGC reduction for strong signals). Again, as in the case of the Line Out, if the audio gain, AGC and RF gain are operated to keep the speaker amplifier out of the clipping range - or away from the compression region just before clipping - the distortion products are entirely acceptable. For those with noisy shacks, impaired hearing or using the internal speaker it may be difficult to find a comfortable listening level without entering the compression region. For those who wish to operate without AGC and with maximum RF gain, I would suggest setting the AF gain to a moderate level and using powered (computer) speakers or a 100 watt per channel stereo amplifier capable of outputs greater than the 1 watt per channel at .1% THD+N specified for the LM4950 with an 8 Ohm load! Note, simply going from 8 to 16 Ohm speakers will improve THD by 8 dB. 73, ... Joe, W4TV _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
