Dealing with a Cheap, Bad, Phono Preamp - Page 1
Forum section: electronics / repair

author: 128ITSH
author_rank: Contributor
timestamp: November 04, 2019, 02:22:06 pm
content: Hello everyoneMy father likes collecting vinyl records and we have an old turntable the we like listening to sometimes.We used to have a good preamp for it, but it stopped working properly so my father orderedthis preamp.We plugged it in and listened to some records and the music just didn't sound good. The treble was very low and that created a feeling of a muffled sound.So I checked the frequency response with my PicoScope 2205A (would recommend) USB oscilloscope and wave generator and the result is as follows:Input (5mV sine) | Output(Avarage RMS)20Hz----------------441mV100Hz---------------730mV200Hz---------------735mV500Hz---------------683mV1kHz----------------565mV2kHz----------------385mV5kHz----------------190mV10kHz--------------115mV20kHz--------------72mVThe eBay page claims "Frequency response: 20Hz-20KHz", but according to my measurements at 20Khz the voltage is amplified 10 times less than the maximum at around 200Hz, and the -3db point seems to be at around 2.2kHz.My questions are:-Is there any way to "fix" such bad preamp by replacing low quality or wrong value components?-Do you think it is possible to get a refund? (i.e. do you have experience with returning similar items to sellers on ebay?)Thanks in advance!

author: m3vuv
author_rank: Super Contributor
timestamp: November 04, 2019, 02:25:36 pm
content: whats the betting its some cheap piece of chineese crap?

author: mansaxel
author_rank: Super Contributor
timestamp: November 04, 2019, 02:48:21 pm
content: Quote from: 128ITSH on November 04, 2019, 02:22:06 pmAre you aware of the RIAA curve? A phono preamplifier should NOT measure linear.Start reading here.Graham Slee makes nice RIAA preamps. I own one.

author: 128ITSH
author_rank: Contributor
timestamp: November 04, 2019, 03:52:37 pm
content: Thanks for the replay.What I understand from this curve is that the record is played with low bass and high treble and to compensate the preamp boosts the bass and lowers the treble.In What way can I measure the quality of this? Maybe looking at FFT spectrum of the input and output while the record is played?

author: mansaxel
author_rank: Super Contributor
timestamp: November 04, 2019, 04:20:20 pm
content: Yes, the compensation curve does two things, mainly:1. By padding the high frequencies, reduces treble noise that comes from playback procedure. This is compensated with a corresponding gain before cutting.2. This is more about the cutting procedure, but still, of course requires compensation. By reducing low frequency amplitude, the pitch (how much the needle is moved inwards per revolution) of the engraving machine can be reduced, and more program material can be fitted onto one side of the disc, without sacrificing maximum level. This is then restored at playback.You need to do a couple of things:Using the RIAA curve as guide, measure gain at the neutral point, 1KHz. Then, measure at octave bands up and down from that and calculate if gain is correct. That will tell you if the compensation curve is b0rked.Buy a better RIAA amplifier!Edit: Or build one. Google tells me there are several reference designs that seem reasonably good.

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 04, 2019, 04:23:14 pm
content: What type of pickup are you using?If I recall correctly (as it has been several decades since I last played a vinyl record), magnetic type pickups require a proper termination.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: November 04, 2019, 04:26:03 pm
content: Quote from: 128ITSH on November 04, 2019, 03:52:37 pmto test the frequency response of your phono preamp, you feed it with an "inverse RIAA"-signal. here you go:https://web.archive.org/web/20170821083136/http://www.hagtech.com/pdf/riaa.pdf

author: mikerj
author_rank: Super Contributor
timestamp: November 04, 2019, 05:00:13 pm
content: If you plot the response of your amp against the desired RIAA equalisation, it's not too far off above 1kHz, but miles out below that.Note that RIAA equalisation is intended only for magnetic cartridges. If you have a ceramic cartridge fitted to your turntable then no equalisation should be applied, and this pre-amp would cut the high frequencies considerably.

author: John.S
author_rank: Contributor
timestamp: November 05, 2019, 09:51:13 am
content: Hi,Maybe asking the obvious, but is your player element indeed a MM (moving magnet) and not a MC ( Moving coil) ?gr. John.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: November 05, 2019, 10:16:32 am
content: Quote from: John.S on November 05, 2019, 09:51:13 amdoesn't matter, equalization is same for both MM and MC, just different gain requirements. MC needs approx. 20 dB more gain at 1 kHz than MM to get same volume level.

author: Gyro
author_rank: Super Contributor
timestamp: November 05, 2019, 12:46:41 pm
content: Quote from: schmitt trigger on November 04, 2019, 04:23:14 pmIrrc, it was 47k || 220pF, although some Ortofon MM cartridges required 470pF. Different for MC.

author: cvanc
author_rank: Frequent Contributor
timestamp: November 05, 2019, 01:41:08 pm
content: Quote from: 128ITSH on November 04, 2019, 02:22:06 pmPerhaps the best answer is to talk to us about your original preamp. We can probably help you fix it.

author: cliffyk
author_rank: Frequent Contributor
timestamp: November 06, 2019, 12:43:56 am
content: Here's the response curve of a Pyle PP555 preamp I recently acquired:Note that it's pretty close to the measurements you made (here are your numbers converted to ΔdB v 2 kHz)ik.e. pretty much what I'd expect from a lower cost RIAA preamp, with a teeper "rumble" cut than the Pyle (which could benefit from a bit more)...

author: floobydust
author_rank: Super Contributor
timestamp: November 06, 2019, 03:33:00 am
content: From the eBay ad pic, it seems to be a 4-transistor circuit with a boost-converter.I would find another preamp, Walt Jung's two op-amp circuit works OK.Problem is many phono-preamps are in the "audiophool" zone so they charge crazy high prices or get silly esoteric. There is a lot of junk out there.Better off to fix the old one.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 06, 2019, 04:15:54 am
content: Might look at some other simple possibilities.Maybe too much gain in new pre-amp and clipping is happening. You can use your Picoscope to look at the outputs for this. This would be almost surely true if the cartridge is ceramic, and adding the RIAA curve would further foul up the sound.The old preamp was failing before it quite completely, and you got used to the sound. The new amp may be sort of correct, but not meeting your preferences.Frequency response of amplifier may depend on load impedance. Typical audio inputs are a few tens of kohms. Picoscope is 1 Mohm.

author: mikerj
author_rank: Super Contributor
timestamp: November 06, 2019, 12:52:02 pm
content: Quote from: floobydust on November 06, 2019, 03:33:00 amIt's must be ok since it's a reasonable circuit layoutThen again it says it's lossless, which doesn't bode (see what I did there?) well for a filter.

author: 128ITSH
author_rank: Contributor
timestamp: November 08, 2019, 12:35:05 pm
content: Thank's for all of your responses!As some suggested I went and checked the old "failed" preamp.Surprisingly, it just works really well. I don't even remember what was the exact issue with it at the first place.Anyways thanks for the help, I learned a few things in this process.