yes you can do it.It calls passive Preamp.Its cost about 30 bucks for 2 separated left and right volume.
Can a simple DIY volume knob replace a pre-amp?
If I have only a cd (single source, can I make a simple volume knob instead getting a pre? Since it is simpler and "pure", will the sound better? I know Pre-amp will do some gain on the signal, what is the benefit to have that gain? I am shopping for a pre-amp and come across some people said that a simple volume can replace the pre. Thanks.
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Yes it is possible. Whether it sounds better than a active preamp ..depends on the rest of the system and your taste. Here's some info sent to me by a designer of tube equipment and transformers.This unit maybe better than the basic passive attenuator. It's also inexpensive to build. Passive Preamplifier - No longer an Oxymoron An introduction into amplification with passive devices. By Jack Elliano - Electra-Print Audio Co. INTRODUCTION Line amplifiers are most commonly used to increase level of a CD player or phono preamp into the amplifiers. Line amps do not have to amplify very much to accomplish this. The average overall gain needed is about four. Normally the job is done with an active device such as a tube with its conventional means to offer a low impedance output to the power amplifiers with standard interconnects. A not so popular means to increase gain is with the use of a step up transformer plus some sort of a variable gain control. This usually fails due as a result of the output impedance being too high for the interconnect cable capacity and attenuation of high frequencies. However, step up transformers using a low impedance source signal can offer a usable stepped up impedance with very little affect from interconnect cable capacity. One saving grace that has become more in popular with music source devices is the very low output impedance of about 50 to 100 ohms. New computer based, hard drive music delivery systems use a pro audio sound card that develop an extremely linear sound, using standard 50 ohm balanced and unbalanced outputs. TRANSFORMER DESIGN One such step up transformer arrangement that will work with op amp outputs must have certain characteristics that will assure full bandwidth, constant load and low distortion. It's primary winding must have a low DCR (Direct Current Resistance) and enough inductance to react with the low end. This parameter is necessary because the DCR is added to the total impedance and is a loss. Due to the very low voltage and power at this impedance, a core material must be used that will increase inductance and move fast over zero crossing at low levels. Nickel laminations work very well at these levels and offer very high permeability. This transformer will also need a high coupling, low capacity wind. We tried a few arrangements of step up and settled with a 1 to 8 voltage gain which offered an impedance of 10,000 ohms from 150 ohm source. This worked well with a 10,000 ohm high quality audio taper control as a constant load and a means to vary the gain for the amplifiers. We calculated that 80khz is the -3db point using a 50pf interconnect from 10,000 ohms source. This seemed safe enough. Note that a 400pf interconnect from a 10,000 ohm source at 20khz will give a reactance of 19,000 ohms so a rolloff of about -2db would be measured. When reactance is equal to source impedance, this is -3db. The transformer we built, and used in this circuit, had an overall bandwidth -1db from 6hz to 55khz. Bandwidth was achieved with 49% nickel core and proper longitudinal balanced windings. The 10K secondary load forced a constant impedance back to the source to operate with. This offered good operational characteristics. WHAT WORKS AND WON'T WORK The input impedance of a common tube amplifier is mostly the value of its input tube grid resistor. This impedance does not have to be matched, it is not transferring power. If it is too low, less than 20K, it may lower the total gain of the PVA (Passive Voltage Amplifier, our name for this) output. The input sensitivity of the amplifier is the most important parameter needed. Sensitivities of .25 to 1 volt will work good and the higher input levels may not give full power output. The output impedance of the PVA is the sum of the resulting transformer secondary and resistance used within the volume control. Some music sources may not work with the this PVA design due to there higher output impedance’s. One remedy for this is to wind a different ratio step up to offer lower gain levels. The CD players with cathode follower outputs most likely are about 3000 ohms so the voltage level should be sufficient to drive an attenuator only, to amplifier input. It would benefit the waveform to pass it through a 1:1 ratio transformer and its nickel core. These type transformers can be custom wound to accommodate any ratio needed. Due to the small size, nickel core and levels involved, a wide bandwidth is always a result. MEASURING OUTPUT IMPEDANCE One means to measure output impedance of a CD player, with close results, is to have a test CD with a 1Khz signal, play it and measure the open unloaded, output signal on a good AC voltmeter. Then add a variable resistance across it, adjust it until the value is half of the open measurement, remove this resistance and measure its resistance with a standard ohmmeter. That value should be very close to the source impedance, at least at 1Khz. SIMPLE CIRCUIT DESCRIPTION As the circuit diagram shows, the volume control wiper then goes to the output, and if needed to sub woofer outputs. The 1k resistor between these outputs are to isolate any low pass filter effects from the normal output. The circuit arrangement here assures that the bandwidth will not change, no matter what volume level you use. It will not add or subtract inductance and capacity as other tapped ratio devices will. The output or source impedance of this device is within 10K or so. It is best to keep the output to the amplifier interconnect capacity, around 50pf. If component isolation is necessary due to a ground loop or if a hum develops while interconnecting unit, the input jacks must be insulated from case. Remove the ground from the input winding or jack. Grounds on the output and bass output must be common and cannot be isolated from each other. LISTENING NOTES The audible result of this device is very interesting and different. One noticeable difference is the increase of amplitude range from low level to high levels as well as increased detail. It can be said that with a nonactive or passive means to develop amplification has an extremely accurate transfer characteristic result. A tube, as many people experience, will add its signature to the sound. Obviously, with no power supply, this design will have no power supply noise residue despite having gain. Normal listening level ranged from 11:00 o'clock to 2:00 o'clock on the control so it has plenty of gain. The silver secondary version of the transformer sounded the same as the all copper but a slight but noticeable added softness or silky sound. Silver is more sensitive to flux variations than copper, it may reveal very high number harmonics. One added comment is that a nickel core low level audio transformer seems to have an interesting resulting sound. The first thing noticed is a wide swing in dynamic range very different from the original CD player opamp output. The second is a clean, rich delivery of harmonics of strings and brass. It was also noted that it will bring out a poorly recorded CD. We know little at this point as to why these are happening but our theories are piling up. CONSTRUCTION The PVA should be built with a steel chassis for best results. Steel will absorb stray magnetic fields from power transformers and chokes in other equipment located nearby. Switching to other inputs can be added to the design in order to accommodate other music sources provided source output impedance’s are between 50 and 150 ohms. Do not exceed 3 volts input or the nickel core will saturate and distortion will occur at low frequencies. PARTS LIST 2 - PVA-3 Electraprint transformer (nickel laminations with copper wire - $51 each) or PVA-3S Electraprint transformer (nickel laminations with silver secondary - $250 each) 1 - 10k audio taper volume pot or equivalent 1 - 7"x5"x2" steel chassis with bottom plate or equivalent 6 - RCA jacks 2 - 1k-1/2 watt resistors |
What you are refering to is called a "passive preamp", which is nothing more than just a potentiometer, aka volume control, stuck in a box of some sort, with RCAs in and out. I know a lot of guys using them, and they do work well within their limitations. Depending on the value of the pot, input capacitance of the amp and cables, blah, blah......you can lose some of the top end. Bottom end usually suffers some, as drive impedance is high. (Don't ask me to explain why, just trust me.) You may like it, you may hate it. Won't cost much to try it. |
I know most of the better pre amps don't use a volume pot or know because it effects the sound as stated above. The better way to do this is with a multiposition slide switch and a bunch of resisters. This is what AR does int he sp-16 pre amp. Most now have electronic controls. There is a reason these things cost bucks it is usally not that simple. But all depends on the quality you want. |
Hi Waip7 , You can call Jack Elliano of Electra print. He will explain to you how to do it. And where the parts can be bought.He makes his own transformers .He's a great guy to call just to pick his brain.He really knows his stuff. He's retired ..so this stuff he does for fun. He will try to save you as much money as possible without steering you down the road of over hyped over priced equipment. Cheers |
You can get high grade hand made stepped attenuators here Endler attenuators. The cost is relatively cheap compared to other companies but the quality is second to none.You just have to know what to look for in this hobby to keep from getting ripped off.Using these in the PVA should give great results if the cd players output impedance is low enough. |
There are 3 basic analog volume controls. Potentiometers are the least expensive but capacitance and inductance can affect the sound. There are some well regarded brands such as Alps. Attenuators use individual resistors and can be anything from simple rotary (step or ladder) to remote and semiconductor controlled (usually not for DIY). These are usually better than pots but can be difficult and expensive to assemble. The exception might be DACT units. The main drawbacks of resistance based units is reducing dynamics at lower settings and the requirement for low output impedance of the source. Placette is highly regarded due to high quality parts. The upside of passive "preamps" is incredible transparency. The third and newest alternative is transformer volume controls (TVC). The Electraprint is the least expensive I've seen. Congratulations, if it works. There's also Sowter(IAG) or S&B (Bent Audio). These also require equipment matching but there is no loss in dynamics. I built one, as an experiment, for less than $1000 and it blew my reference quality tube preamp right off the rack. The other nice thing about TVC's is that they are completely indifferent between balanced and single-ended (autoformers excluded). Of course, there's always Aunt Corey's but the BUF-03 has been discontinued. http://www.stereophile.com/amplificationreviews/54/index1.html |
As a relative newcomer to TVCs (transformer volume control), and having recently acquired the Bent Audio NOH (silver transformer)I can unreservedly endorse all that Ngjockey has said in the above post. As an audiophile once remarked, it is "insultingly neutral". There is absolutely no loss in dynamics. The realism of brass, strings and subtle details is absolutely unbelievable. Voices are so life-like, I have found it very easy to actually hear singers articulate the lyrics to songs so much easier. Dan Fogelberg's "River of Souls" (track 3 - The Minstrel) has a passage with percussion (sounds like pins tapping on cymbals) that has hitherto been eluded by my other active preamps (high end I might add)but it shone with the Bent NOH.Ross Mantle of Ultra Audio wrote a seminal article in June 2003 with the provocative title, "Quick everyone, Sell your Preamps", extolling the virtues of the AVTAC Pasiphae preamp which uses Stevens & Billington's TVC. More recently, 6Moons and other audio journals are showing renewed interest in TVCs. Audio Zone's PRET-1, Music First's Magnetic passives and Bent Audio's NOH TVCs - no doubt many more will follow - shows that there is a better alternative to active preamps and might perhaps foretell that the future of preamps is to have no preamps but a TVC - no extra power cord, no resistors in its path. Can you imagine the future of an integrated amplifier with a TVC? Don't get me wrong: I am not saying TVCs are for everyone! it requires careful matching of source output and amplifier input, but if you can handle the absolute TRUTH there is nothing that can bring you closer to the honest truth than TVCs. |
I don't mean to thread hi jack but I'd love to know if I can continue my couch potato stance on never getting up unless I need to turn my stuff off. I guess this may need to be a whole other thread but I thought that since I've got some DIY gurus here I'd pose the question on how, where and what to do. TIA and sorry for the hi jack, hope you get your passive pre taken care of! Happy Holidays! |